neighbour: Remove unused function declaration pneigh_for_each()

pneigh_for_each() is never implemented since the beginning of git history.
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/neighbour.h

@@ -394,8 +394,6 @@ void neigh_for_each(struct neigh_table *tbl,
 void __neigh_for_each_release(struct neigh_table *tbl,
 			      int (*cb)(struct neighbour *));
 int neigh_xmit(int fam, struct net_device *, const void *, struct sk_buff *);
-void pneigh_for_each(struct neigh_table *tbl,
-		     void (*cb)(struct pneigh_entry *));
 
 struct neigh_seq_state {
 	struct seq_net_private p;