_git: git rm: make git rm --cached work as intended

While limiting the completion to __git_changed-in-index_files
makes perfect sense for "git diff", it is wrong to use for
"git rm", because it prevents it from completing files present in
HEAD. Giving --cached to git rm is supposed to
"just remove it from the index, keep the working tree copy"
and is in no way related to the file needing to be
modified in the index in the first place.