From the Magit docs:
Furthermore the old dependencies
git-commit-modeandgit-rebase-modehave to be removed because they are no longer used by the2.1.0release and later, and get in the way of their successorsgit-commitandgit-rebase.
I have a lot of machines that have these old packages lying around. Is there any way I can teach magit to automatically clean them up?