netfilter: remove useless prototype

prototype nf_ct_nat_offset is not used anymore.
Signed-off-by: Taehee Yoo <ap420073@gmail.com>

include/net/netfilter/nf_conntrack.h

@@ -213,11 +213,6 @@ static inline bool nf_ct_kill(struct nf_conn *ct)
 	return nf_ct_delete(ct, 0, 0);
 }
 
-/* These are for NAT.  Icky. */
-extern s32 (*nf_ct_nat_offset)(const struct nf_conn *ct,
-			       enum ip_conntrack_dir dir,
-			       u32 seq);
-
 /* Set all unconfirmed conntrack as dying */
 void nf_ct_unconfirmed_destroy(struct net *);