netfilter: conntrack: remove helper hook again

place them into the confirm one.

Old:
 hook (300): ipv4/6_help() first call helper, then seqadj.
 hook (INT_MAX): confirm

Now:
 hook (INT_MAX): confirm, first call helper, then seqadj, then confirm

Not having the extra call is noticeable in bechmarks.

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