netfilter: conntrack: remove remaining l4proto indirect packet calls

Now that all l4trackers are builtin, no need to use a mix of direct and
indirect calls.
This removes the last two users: gre and the generic l4 protocol
tracker.

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