Allow removal of the Gitaly Cluster database state without removal of the on-disk repository

In some cases we may end up with repos that are present only in the DB that need to be cleaned up. These can be removed with praefect remove-repository, but admins may accidentally pass in a healthy repo to this command and cause data loss.

To give extra assurance, a --database-only flag that will only proceed if the repo not present on disk would make this operation safer.

Release notes

Documentation - https://docs.gitlab.com/ee/administration/gitaly/recovery.html#manually-remove-repositories

In certain situations you may want to manually remove the Gitaly Cluster database entry while leaving the on-disk repository intact. We now allow for this by specifying the -db-only flag. This enables administrators to remove an orphaned database entry while protecting the on-disk repository.

If you want to re-track a repository, you can use the track-repository command documented here.