netfilter: conntrack: remove unused init_net hook

since adf0516845 ("netfilter: remove ip_conntrack* sysctl compat code")
the only user (ipv4 tracker) sets this to an empty stub function.

After this change nf_ct_l3proto_pernet_register() is also empty,
but this will change in a followup patch to add conditional register
of the hooks.

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

include/net/netfilter/nf_conntrack_l3proto.h

@@ -63,9 +63,6 @@ struct nf_conntrack_l3proto {
 
 	size_t nla_size;
 
-	/* Init l3proto pernet data */
-	int (*init_net)(struct net *net);
-
 	/* Module (if any) which this is connected to. */
 	struct module *me;
 };