netfilter: Refactor nf_conncount

Remove parameter 'family' in nf_conncount_count() and count_tree().
It is because the parameter is not useful after commit 625c5561
("netfilter: connlimit: split xt_connlimit into front and backend").

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>