netfilter: remove xt pernet data

clusterip is now handled via net_generic.

NOTRACK is tiny compared to rest of xt_CT feature set, even the existing
deprecation warning is bigger than the actual functionality.

Just remove the warning, its not worth keeping/adding a net_generic one.

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