Using LaTeX Beamer for a presentation, does anyone know how to remove the border from the frame title on the references slide?

** Updated question with MWE

I am currently using a custom background template image and at the end of my presentation, I want to display the citations that I used. However, I cannot seem to figure out how to eliminate the black lines around the top, left & right side of the frame title "References".

Here is the text used to create the background template from a PDF

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                  XXX PhD Presentation                                  %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\documentclass[xcolor=dvipsnames,aspectratio=1610,handout]{beamer} % handout
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                                   Latex Preamble                              %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\usepackage[utf8]{inputenc}
\usetheme[block=fill,progressbar=frametitle]{metropolis}
\setbeamertemplate{frame numbering}[fraction]
\usepackage{appendixnumberbeamer}
\usepackage[sort&compress,numbers]{natbib} % citations
\usepackage{tikz}
\usepackage{tcolorbox} % Use for nice backgrounds around code
\usepackage{enumerate}
\usepackage{amsmath,mathtools,cancel}
\usepackage{media9} %pdflatex, latex+dvips+ps2pdf, xelatex # Allows for the Youtube video

\usepackage{minted}  % Code highlighting
\usemintedstyle[python]{perldoc}
\usemintedstyle[matlab]{colorful}
\tcbuselibrary{breakable,skins,minted}

\usefonttheme[onlymath]{serif}

% Specific Colors
\definecolor{URed}{HTML}{CC0000}
\definecolor{UBlack}{HTML}{000000}
\definecolor{UGray}{HTML}{808080}
\definecolor{paperColor}{HTML}{e2c88e}
\definecolor{pythoncodebg}{rgb}{0.99,1,0.99}
\definecolor{matlabcodebg}{rgb}{0.99,0.99,1}

% Algorithm writing
\usepackage{algpseudocode}
\usepackage{algorithm}

% Colored letters around objects
\newcommand\encircleP[1]{%
    \tikz[baseline=(X.base)] 
    \node (X) [draw, shape=circle, inner sep=0, fill=red, text=black] {\strut #1};%
}
\newcommand\encircleA[1]{%
    \tikz[baseline=(X.base)] 
    \node (X) [draw, shape=circle, inner sep=0, fill=yellow, text=black] {\strut #1};%
}

%%%%%%%%%%%%% Beamer specific attributes %%%%%%%%%%%%%%%%%%%%
\setbeamertemplate{section in toc}[sections numbered]
\setbeamersize{text margin left=20pt,text margin right=20pt}

% Add support for \subsubsectionpage
\def\subsubsectionname{\translate{Subsubsection}}
\def\insertsubsubsectionnumber{\arabic{subsubsection}}
\setbeamertemplate{subsubsection page}
{
    \begin{centering}
        {\usebeamerfont{subsubsection name}\usebeamercolor[fg]{subsubsection name}\subsubsectionname~\insertsubsubsectionnumber}
        \vskip1em\par
        \begin{beamercolorbox}[sep=4pt,center]{part title}
            \usebeamerfont{subsubsection title}\insertsubsubsection\par
        \end{beamercolorbox}
    \end{centering}
}
\def\subsubsectionpage{\usebeamertemplate*{subsubsection page}}

\AtBeginSection{\frame{\sectionpage}}
\AtBeginSubsection{\frame{\subsectionpage}}
\AtBeginSubsubsection{\frame{\subsubsectionpage}}

% Specific beamer colors
%\setbeamercolor{background canvas}{bg=UGray}
\setbeamercolor{progress bar}{fg=URed,bg=UGray}
\setbeamercolor*{sidebar}{fg=URed,bg=UBlack!80!white}
\setbeamercolor{frametitle}{bg=UGray,fg=white}
\setbeamercolor{section title}{fg=UBlack!80!white}
\setbeamercolor{title}{fg=UBlack!80!white}
\setbeamercolor{author}{fg=UBlack!80!white}
\setbeamercolor{institute}{fg=UBlack!80!white}
\setbeamercolor{date}{fg=UBlack!80!white}
\setbeamercolor{section}{fg=UBlack!80!white}
\setbeamercolor{section in toc}{fg=UBlack!80!white,bg=white}

%%%%% Beamer Alerts %%%%%
\setbeamercolor{alerted text}{fg=URed!30!UBlack} % Alerts for emphasizing text in bullets
\setbeamerfont{alerted text}{series=\normalfont} % Adjust the alert for emphasising text
\mode<handout>{
    \setbeamercolor{alerted text}{fg=UBlack!80!white} % Alerts for emphasizing text in bullets
}

% Beamer font size
\setbeamerfont{footnote}{size=\tiny}

% custom heading line in the beamer slide
\newcommand{\heading}[1]{%
    \noindent\textbf{\color{UBlack!80!white}\normalfont\bfseries #1}%
    \vspace{3pt}%
    {\color{URed}\hrule}%
}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Adjust beamer length of itemize lists
\makeatletter
\renewcommand{\itemize}[1][]{%
    \beamer@ifempty{#1}{}{\def\beamer@defaultospec{#1}}%
    \ifnum \@itemdepth >2\relax\@toodeep\else
    \advance\@itemdepth\@ne
    \beamer@computepref\@itemdepth% sets \beameritemnestingprefix
    \usebeamerfont{itemize/enumerate \beameritemnestingprefix body}%
    \usebeamercolor[fg]{itemize/enumerate \beameritemnestingprefix body}%
    \usebeamertemplate{itemize/enumerate \beameritemnestingprefix body begin}%
    \list
    {\usebeamertemplate{itemize \beameritemnestingprefix item}}
    {%
        \setlength\topsep{0pt}%NEW
        \setlength\partopsep{0pt}%NEW
        \setlength\itemsep{0pt}%NEW
        \def\makelabel##1{%
            {%
                \hss\llap{{%
                        \usebeamerfont*{itemize \beameritemnestingprefix item}%
                        \usebeamercolor[fg]{itemize \beameritemnestingprefix item}##1}}%
            }%
        }%
    }
    \fi%
    \beamer@cramped%
    \raggedright%
    \beamer@firstlineitemizeunskip%
}
\makeatother

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Beamer version of cancel to make the arrow red
\renewcommand{\CancelColor}{\color{red}} %change cancel color to red

\makeatletter
\let\my@cancelto\cancelto %copy over the original cancelto command
\newcommand<>{\cancelto}[2]{\alt#3{\my@cancelto{#1}{#2}}{\mathrlap{#2}\phantom{\my@cancelto{#1}{#2}}}}
% redefine the cancelto command, using \phantom to assure that the
% result doesn't wiggle up and down with and without the arrow
\makeatother

%%%%%%  I couldn't get this to cover the entire slide %%%%%%
%\usebackgroundtemplate{%
%   \tikz\node[opacity=0.2] {\includegraphics[height=\paperheight,width=\paperwidth]{./Figures/old_paper_with_Larger_Eye.png}};} % old_paper_with_Larger_Eye

%%%%%%  Alternative approach to cover the background slide %%%%%%
\pgfdeclareimage[width=\paperwidth,height=\paperheight]{mybackground}{./Figures/old_paper_with_Larger_Eye.pdf}

% Use with PdfLaTeX
\setbeamertemplate{background canvas}{%
    \begin{picture}(160mm,100mm)
        \begin{beamercolorbox}[wd=\paperwidth,ht=\paperheight]{bgcolor}
        \end{beamercolorbox}
        \put(-419,-3.5){%
            \tikz\node[opacity=0.15]{\pgfuseimage{mybackground}};
        }
    \end{picture}
}

% Presentation attributes
\title{Presentation Title}
%\subtitle{\small Subtitle}
\author[XXX]{First Name Last Name} % Author
\institute{\small Institute}
\date{\tiny\today}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                                   Latex Preamble                              %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}

% ---------------------------------------------------------------------------- %
\begin{frame}
  \titlepage
\end{frame}
\note{Disclosure statements ``None exist"}

\begin{frame}{Overview}
    \tableofcontents
\end{frame}
% ---------------------------------------------------------------------------- %

\section{Background}
% ---------------------------------------------------------------------------- %

\begin{frame}[t]
    \frametitle{Motivation}
    \heading{Retinal Diseases}
    \begin{itemize}[<+-| alert@+>] \pause
        \item[] Age-related macular degeneration (AMD)
        \begin{itemize}[<+-| alert@+>]
            \item 8 million Americans and anticipated to increase by 50\% by 2020 \cite{Jager_2008}
        \end{itemize}
        \item[] Posterior vitreous detachment (PVD)
        \begin{itemize}[<+-| alert@+>]
            \item 24\% of the population, 50-59 yrs. 87\% 80-89 yrs. \cite{Bond-Taylor_2017}
        \end{itemize}
        \item[] Traumatic rhegmatogenous retinal detachment (RD)
        \begin{itemize}[<+-| alert@+>]
            \item Approximately 1 in 10,000 individuals suffer from retinal detachment each year \cite{Gariano_2004} about 15\% of these are traumatic \cite{Mitry_2010}
        \end{itemize}

    \end{itemize}
\end{frame}

\appendix
{\tiny
\begin{frame}[t, allowframebreaks]
    \bibliographystyle{ieeetr} % Orders the bibliography to begin with the number 1, 2, 3, .
    \bibliography{Dissertation_Presentation_Bib}    
\end{frame}}
\end{document}

When I display the bibliography at the end of my presentation I get three black lines around the edges of the references frame title and I'm not sure how to get rid of them. I want the red line there, just not the three black lines. All other frame titles do not have those additional lines around the text.

Three black lines around the references frame title that I want to remove

Thanks so much!

Solution

Natbib normally start a new section for the references, but starting a new section inside a frame is not a good idea. You can avoid the problem, by redefining the \bibsection macro:

\documentclass[xcolor=dvipsnames,aspectratio=1610,handout]{beamer} 
\usetheme[block=fill,progressbar=frametitle]{metropolis}
\usepackage[sort&compress,numbers]{natbib} % citations

\renewcommand{\bibsection}{}

\begin{document}

\begin{frame}
  \frametitle{title}
  \cite{test}
\end{frame}

\begin{frame}
    \frametitle{References}
    \bibliographystyle{ieeetr} 
    \bibliography{\jobname}    
\end{frame}

\end{document}