duplicated port check in remove_router_interface

In remove_router_interface() in db/l3_db.py, port_db['device_id'] != router_id is done twice.

1st check
    353 if not (port_db['device_owner'] == DEVICE_OWNER_ROUTER_INTF and
    354 port_db['device_id'] == router_id):
    355 raise w_exc.HTTPNotFound("Router %(router_id)s does not have "
    356 " an interface with id %(port_id)s"
    357 % locals())
covers 2nd check
    365 if port_db['device_id'] != router_id:
    366 raise w_exc.HTTPConflict("port_id %s not used by router" %
    367 port_db['id'])

2nd check has no meaning and it should be removed.