netfilter: netns: shrink netns_ct struct

remove the obsolete sysctl anchors and move auto_assign_helper_warned
to avoid/cover a hole.  Reduces size by 40 bytes on 64 bit.

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