net: get rid of rtnl_lock_unregistering()

After recent patches, and in particular commits
 faab39f6 ("net: allow out-of-order netdev unregistration") and
 e5f80fcf ("ipv6: give an IPv6 dev to blackhole_netdev")
we no longer need the barrier implemented in rtnl_lock_unregistering().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>