netfilter: nf_conntrack: remove unused ctl_table_path member in nf_conntrack_l3proto

After commit adf05168 ("netfilter: remove ip_conntrack* sysctl
compat code"), ctl_table_path member in struct nf_conntrack_l3proto{}
is not used anymore, remove it.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>