[NETFILTER]: nf_queue: remove unnecessary hook existance check

We hold a module reference for each queued packet, so the hook that
queued the packet can't disappear. Also remove an obsolete  comment
stating the opposite.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>