Avoid DB errors when deleting network's ports and subnets

Avoid DB errors when deleting network's ports and subnets

DB errors may occur when accessing query results
after the transaction was closed (like ObjectDeletedError).
Hence it's better to avoid DB object access especially
when it's not needed.
This patch changes _delete_ports() and _delete_subnets() to accept
only ids. Indeed, there is no need to pass db objects to these methods.

Closes-Bug: #1484135
Related-Bug: #1454408
Change-Id: I7507cb1c85defb2e6d5144e5832aea713d6251ae