net: Remove unused inline function sk_nulls_node_init()

All uses of sk_nulls_node_init() have
been removed since commit dbca1596 ("ping: convert to RCU
lookups, get rid of rwlock"), so remove it.
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/sock.h

@@ -744,11 +744,6 @@ static inline void sk_node_init(struct hlist_node *node)
 	node->pprev = NULL;
 }
 
-static inline void sk_nulls_node_init(struct hlist_nulls_node *node)
-{
-	node->pprev = NULL;
-}
-
 static inline void __sk_del_node(struct sock *sk)
 {
 	__hlist_del(&sk->sk_node);