bonding: get netdev_rx_handler_unregister out of locks

Now that netdev_rx_handler_unregister contains synchronize_net(), we need
to call it outside of bond->lock, cause it might sleep. Also, remove the
already unneded synchronize_net().

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