Remove more orphans when removing stray namespaces

This adds some extra orphan removals to the migration db/migrate/20161117114805_remove_undeleted_groups.rb, hopefully solving the problem described in https://gitlab.com/gitlab-org/gitlab-ce/issues/25146.

@rdavila is it possible the user in question can test these changes and report back to see if it solves the problem? There may be more orphaned rows that we don't know about.