Also known as
HEAD is not a museum.
The purpose of an SCM system is to store a history of your work, so there's no need to keep unused code or comments in your current build.
Whenever you see commented-out or unused code and/or outdated comments in source code or documentation.
Remove unused variables, methods etc., any code commented-out without an accompanying explanation (e.g. a temporary workaround for a blocking issue), any tests ignored without explanation and any outdated comments or documentation from your source code and update your SCM system. Also, remind your colleagues that these items should be removed.
The HEAD of your project will reflect the current state of your code, not an arbitrary mix or current code any stuff hanging around because nobody is 100% sure whether it is in use or not.