I installed Mercurial, only to realize that it's not supported yet with MonoDevelop, and then realized that I have no clue how to properly uninstall it.
I've googled around and can't find anything to support uninstalling it.
MonoDevelop's support for version control is lagging, and only basic support for Git will be added in version 2.6. I would suggest keeping Hg installed and use the command line. You shouldn't dismiss using a great DVCS just because your IDE doesn't integrate with it.
Since Hg is Python based, you may be able to use easy_install to uninstall it for you.
easy_install -m mercurial
See easy_install documentation for more information