netlabel: Remove unused declaration netlbl_cipsov4_doi_free()

Since commit b1edeb10 ("netlabel: Replace protocol/NetLabel linking with refrerence counts")
this declaration is unused and can be removed.
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20230801143453.24452-1-yuehaibing@huawei.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/netlabel/netlabel_cipso_v4.h

@@ -149,7 +149,4 @@ enum {
 /* NetLabel protocol functions */
 int netlbl_cipsov4_genl_init(void);
 
-/* Free the memory associated with a CIPSOv4 DOI definition */
-void netlbl_cipsov4_doi_free(struct rcu_head *entry);
-
 #endif