Delete refs/merge-requests/:iid/merge when MR is merged

This is a follow-up for https://gitlab.com/gitlab-org/gitlab-ce/issues/57252.

Currently we don't erase the refs/merge-requests/:iid/head (context on https://gitlab.com/gitlab-org/gitlab-ce/issues/36863).

Ideally we should manage to delete refs/merge-requests/:iid/merge when a MR is merged to avoid keeping it in the repo unnecessarily. Unless we have a good reason do keep it.