Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/59777.

In earlier iterations of our implementation of Git object deduplication
we thought we would be making extensive use of Git remotes in pool
repositories in the future, and that we should manage these remotes
carefully from the start. We now expect we only care about one remote,
namely the source project. The other remotes are there only for forensic
purposes.

Before this MR we tried to also remove pool remotes when member projects
got deleted, with the UnlinkRepositoryFromObjectPool RPC. This is
fragile when there are race conditions (see
https://gitlab.com/gitlab-org/gitaly/issues/1568#note_153955926). We
have spent some time making this RPC less fragile in
https://gitlab.com/gitlab-org/gitaly/merge_requests/1151 but looking at
this problem again, I think we should just stop calling it.