netfilter: remove old pre-netns era hook api

no more users in the tree, remove this.

The old api is racy wrt. module removal, all users have been converted
to the netns-aware api.

The old api pretended we still have global hooks but that has not been
true for a long time.

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