netfilter: gre: Remove unused function declaration nf_ct_gre_keymap_flush()

Commit a23f89a99906 ("netfilter: conntrack: nf_ct_gre_keymap_flush() removal")
leave this unused, remove it.

Signed-off-by: Yue Haibing <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>

include/linux/netfilter/nf_conntrack_proto_gre.h

···
 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);