Big Switch Plugin: No REST port delete on net del

Moves the REST call for port deletion out of the
_delete_port method into the delete_port method
so it doesn't get called during a delete_network
operation. The backend controller automatically
removes ports that are a member of a network when
it's deleted.

Closes-Bug: #1295438
Change-Id: Ic09eb722f8e9bd3ce298dee90d8415cda1a1aa7a
Merge "Big Switch Plugin: No REST port delete on net del"