netfilter: remove synchronize_net() calls in ip_queue/ip6_queue

nf_unregister_queue_handlers() already does a synchronize_rcu()
call, we dont need to do it again in callers.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>