netfilter: do not unbind the queues on exit

On exit we were calling nfq_unbind_pf for AF_INET and AF_INET6, which
usually stcuked the daemon for a very long time, and other times
caused a segfault.

According to the docs (..), calling nfq_destroy_queue() is enough to
exit cleanly:
"This call also unbind from the nfqueue handler, so you don't have to
call nfq_unbind_pf."
https://netfilter.org/projects/libnetfilter_queue/doxygen/html/group__Queue.html#ga32a1461e9a36c95a03cb4476109f33bb

Closes #919

daemon/netfilter/queue.go

@@ -163,8 +163,6 @@ func (q *Queue) destroy() {
 		}
 		os.Exit(0)
 	})
-	C.nfq_unbind_pf(q.h, AF_INET)
-	C.nfq_unbind_pf(q.h, AF_INET6)
 	if q.qh != nil {
 		if ret := C.nfq_destroy_queue(q.qh); ret != 0 {
 			log.Warning("Queue.destroy() idx=%d, nfq_destroy_queue() not closed: %d", q.idx, ret)