Remove remaining recursive queries on repo delete and add test

test/test_gc.py

@@ -121,7 +121,7 @@ class TestGarbageCollection(unittest.TestCase):
     """
     # Delete all existing repos.
     for repo in database.Repository.select():
-      repo.delete_instance(recursive=True)
+      model.repository.purge_repository(repo.namespace_user.username, repo.name)
 
     # Change the time machine expiration on the namespace.
     (database.User.update(removed_tag_expiration_s=1000000000)