Alter old migrations so that they can downgrade

The Role -> User foreign key constraint was preventing roles from being
deleted once there were existing users referencing the role. I realized
it was best to just pass on the downgrade and allow the tables to be
deleted.

alembic/versions/4ea5917e7781_add_default_atat_role.py

@@ -34,6 +34,4 @@ def upgrade():
 
 
 def downgrade():
-    db = op.get_bind()
-    db.execute("DELETE FROM roles WHERE name = 'default'")
-
+    pass