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>