netfilter: revert a2361c87

This patch reverts a2361c87

:
"[PATCH] netfilter: xt_conntrack: warn about use in raw table"

Florian Wesphal says:
"... when the packet was sent from the local machine the skb
already has ->nfct attached, and -m conntrack seems to do
the right thing."

Acked-by: Jan Engelhardt <jengelh@medozas.de>
Reported-by: Florian Wesphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>