netfilter: conntrack: remove l4proto destroy hook

Only one user (gre), add a direct call and remove this facility.

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

include/net/netfilter/nf_conntrack_l4proto.h

@@ -27,9 +27,6 @@ struct nf_conntrack_l4proto {
 	/* protoinfo nlattr size, closes a hole */
 	u16 nlattr_size;
 
-	/* Called when a conntrack entry is destroyed */
-	void (*destroy)(struct nf_conn *ct);
-
 	/* called by gc worker if table is full */
 	bool (*can_early_drop)(const struct nf_conn *ct);