Revert "net: fix unused variable warning in do_tcp_setsockopt()"

This reverts commit 7abf8b09720539a0c4f2561eff4002278e8b27c0.

Reason for revert: Android common kernel backported the related changes from 4.19
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 25fded7..b6a3a40 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2511,6 +2511,7 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct net *net = sock_net(sk);
 	int val;
 	int err = 0;