[NETFILTER]: nf_queue: remove unused data pointer

Remove the data pointer from struct nf_queue_handler. It has never been used
and is useless for the only handler that really matters, nfnetlink_queue,
since the handler is shared between all instances.

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