I thought I was going to use Box2d in my game but I won't use it anymore. How can I remove the extension? I was finding if someone has already made this question, but I didn't find.
This is very simple. You will only have to remove the dependencies from your projects.
Open your build.gradle
file in the root folder of your projects and search for these dependencies.
After that you will probably have to trigger a refresh in your IDE. For example in eclipse you do this via a rightclick in your project -> Gradle -> Refresh dependencies.