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