Fix empty network deletion in db_base_plugin for postgresql

Need to disable joined loads for port query in delete_network
as in other methods.

No unit test is being added, because the problem is not reproducible with
sqlite.

Closes-Bug: #1261652

Change-Id: I7a015be6f2f9b8ae43bc29a767d16af30120f13f
Merge "Fix empty network deletion in db_base_plugin for postgresql"