sync callbacks: Eliminate SUBNET_GATEWAY resource
sync callbacks: Eliminate SUBNET_GATEWAY resource

Commit 7b239a928f7834ff98e0fff82491bfbb2a10eece removed subnet gateway
from the callback resources in neutron. This follows suit in lib in
order to sync up.

Change-Id: I7b2f2f018723fed6c30480a92dc89654c352e6ae

openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib