netfilter: tproxy: remove nf_tproxy_core, keep tw sk assigned to skb

The module was "permanent", due to the special tproxy skb->destructor.
Nowadays we have tcp early demux and its sock_edemux destructor in
networking core which can be used instead.

Thanks to early demux changes the input path now also handles
"skb->sk is tw socket" correctly, so this no longer needs the special
handling introduced with commit d503b30b
(netfilter: tproxy: do not assign timewait sockets to skb->sk).

Thus:
- move assign_sock function to where its needed
- don't prevent timewait sockets from being assigned to the skb
- remove nf_tproxy_core.

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