delete share network subnet on share network deletion

When share network is deleted, we check if only 1 share network subnet is associated with share network. But we end up deleting share network in db without deleting that single share network subnet associated with share network. This results in zombie share network subnet entries in db.

We used below query and it returns non-zero result.

MariaDB [manila]> SELECT COUNT(id) FROM share_network_subnets WHERE deleted='False' AND share_network_id IS NULL;