netfilter: remove nf_ct_unconfirmed_destroy helper

This helper tags connections not yet in the conntrack table as
dying.  These nf_conn entries will be dropped instead when the
core attempts to insert them from the input or postrouting
'confirm' hook.

After the previous change, the entries get unlinked from the
list earlier, so that by the time the actual exit hook runs,
new connections no longer have a timeout policy assigned.

Its enough to walk the hashtable instead.

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