[NETFILTER] Remove nf_conntrack stat proc file when cleaning up

Fix nf_conntrack statistics proc file removal. Looks like the old bug
was forward-ported from ip_conntrack. :-]

Signed-off-by: KOVACS Krisztian <hidden@balabit.hu>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>