operations: Fix deletion of branches with `refs/heads` prefix

When deleting branches, we currently still have a Ruby bug-for-bug compatibility layer in place which strips the refs/heads prefix of a branch name in case the user provided it. So if the user intends to delete refs/heads/foobar, we'd try to delete the reference refs/heads/foobar instead of refs/heads/refs/heads/foobar.

Now that the Ruby code doesn't exist anymore, let's also drop the compatibility layer and use sane behaviour here: if the user tells us to delete a branch refs/heads/foobar, we delete a branch refs/heads/foobar.