Remove unused method from network rpcapi

Remove get_vifs_by_instance from the nova network rpcapi.  It's unused,
so we can go ahead and remove the client side of this.  The server
(manager) side can be removed on a major rpc api version bump.

Change-Id: Iab90e87e7a82abcf290c2fc799001cc4ee8299b2
Merge "Remove unused method from network rpcapi"