netfilter: core: remove synchronize_net call if nfqueue is used

since commit 960632ec

 ("netfilter: convert hook list to an array")
nfqueue no longer stores a pointer to the hook that caused the packet
to be queued.  Therefore no extra synchronize_net() call is needed after
dropping the packets enqueued by the old rule blob.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>