bridge: remove a redundant synchronize_net()

commit 00cfec37 (net: add a synchronize_net() in
netdev_rx_handler_unregister())
allows us to remove the synchronized_net() call from del_nbp()

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