netfilter: gre: Remove unused function declaration nf_ct_gre_keymap_flush()

Commit a23f89a9 ("netfilter: conntrack: nf_ct_gre_keymap_flush() removal")
leave this unused, remove it.
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Florian Westphal <fw@strlen.de>

include/linux/netfilter/nf_conntrack_proto_gre.h

@@ -25,7 +25,6 @@ struct nf_ct_gre_keymap {
 int nf_ct_gre_keymap_add(struct nf_conn *ct, enum ip_conntrack_dir dir,
 			 struct nf_conntrack_tuple *t);
 
-void nf_ct_gre_keymap_flush(struct net *net);
 /* delete keymap entries */
 void nf_ct_gre_keymap_destroy(struct nf_conn *ct);