netfilter: kill NF_HOOK_THRESH() and state->tresh

Patch c5136b15 ("netfilter: bridge: add and use br_nf_hook_thresh")
introduced br_nf_hook_thresh().

Replace NF_HOOK_THRESH() by br_nf_hook_thresh from
br_nf_forward_finish(), so we have no more callers for this macro.

As a result, state->thresh and explicit thresh parameter in the hook
state structure is not required anymore. And we can get rid of
skip-hook-under-thresh loop in nf_iterate() in the core path that is
only used by br_netfilter to search for the filter hook.

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