tcp: Remove the unused api

Last tcp_write_queue_head() use was removed in commit
114f39fe ("tcp: restore autocorking"), so remove it.
Signed-off-by: Tao Chen <chentao3@hotmail.com>
Link: https://lore.kernel.org/r/SYZP282MB33317DEE1253B37C0F57231E86029@SYZP282MB3331.AUSP282.PROD.OUTLOOK.COMSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/tcp.h

@@ -1831,11 +1831,6 @@ static inline struct sk_buff *tcp_rtx_queue_tail(const struct sock *sk)
 	return skb_rb_last(&sk->tcp_rtx_queue);
 }
 
-static inline struct sk_buff *tcp_write_queue_head(const struct sock *sk)
-{
-	return skb_peek(&sk->sk_write_queue);
-}
-
 static inline struct sk_buff *tcp_write_queue_tail(const struct sock *sk)
 {
 	return skb_peek_tail(&sk->sk_write_queue);