Delete subnet fails if assoc port has IPs from another subnet

This change fixes the following failure scenario:
- Create a network
- Create two subnets on the network
- Create a port on the network using one of the subnets
- Delete the other subnet
= = = > FAILURE: Subnet delete fails because supposedly there
is/are port(s) still associated with that subnet.

The problem addressed is that delete_subnet() in
neutron/db/db_base_plugin.py is checking for port(s) still being
associated with the subnet's network, not the subnet itself.

Change-Id: I7adbe18cce412135b2e42dcb7c592e60c1ec5f8f
Closes-Bug: #1281694
Merge "Delete subnet fails if assoc port has IPs from another subnet"