gitlab: Do away with generating tarball backups

Using GitLab's official backup tool takes too much time and, more importantly, space; /srv/gitlab is a bit over 430G but backing it up nearly exhausts its 1TB volume.

As we're creating btrfs snapshots and backing those up with borg, it seems unnecessary to also create tarballs of the same data. GitLab's documentation mentions snapshots as a viable backup strategy, and to the restored system it should seem like recovering from a power loss.

[1] https://docs.gitlab.com/ee/raketasks/backup_restore#alternative-backup-strategies

docs/backups.md

@@ -85,10 +85,6 @@ For PostgreSQL backups are made using pg_dump to `postgres_backup_dir`.
 
 Restoring backups can be done with `pg_restore`. See also [official PostgreSQL docs](https://www.postgresql.org/docs/current/app-pgrestore.html).
 
-### Gitlab
-
-GitLab is backupped up using the `gitlab_backup` tool to `gitlab_backupdir`. See also [official GitLab docs](https://docs.gitlab.com/ee/raketasks/backup_restore.html).
-
 ## Adding a new server
 
 Adding a new server to be backed up goes as follows: