DF L3 service plugin does not need agent RPC
DF L3 service plugin does not need agent RPC

The DF L3 service plugin inherits from the neutron
ExtraRoute_db_mixin and L3_NAT_db_mixin class which
eventually inherits from the neutron L3RpcNotifierMixin
class. The L3RpcNotifierMixin inheritance isn't
necessary and adds unneeded RPC overhead since the DF
L3 service plugin doesn't use an L3 agent RPC.

Change-Id: I6f0fc1e6e7250d62b6a77eecd63576d9c37f6bfc

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow