Phonet: remove useless locking in device cleanup

Incoming packets and sockets are already gone.
The netdevice notifier is unregistered under the RTNL lock
There remains a race with the rtnetlink handlers unregistration, but it
is a generic RTNL issue that was already present before this change.

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>