Stop removing ip allocations on port delete

The _delete_port() method was manually removing related
IPAllocation instances despite the existence of a perfectly
good cascade deletion relationship in the model.  This patch
puts an end to that nonsense and the potential for deadlock that
it represented.

Closes-bug: #1288379
Related-Bug: #1283522

Change-Id: Ib31550fa9000fc75768a327cb6cc1c419e06568f
(cherry picked from commit 2648aa3561d23e1215e0cc6f446253e5df56c8f6)
Merge "Stop removing ip allocations on port delete" into stable/havana