tcp: remove set but not used variable 'skb_size'

Fixes gcc '-Wunused-but-set-variable' warning:

net/ipv4/tcp_output.c: In function 'tcp_collapse_retrans':
net/ipv4/tcp_output.c:2700:6: warning:
 variable 'skb_size' set but not used [-Wunused-but-set-variable]
  int skb_size, next_skb_size;
      ^
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -2698,9 +2698,8 @@ static bool tcp_collapse_retrans(struct sock *sk, struct sk_buff *skb)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct sk_buff *next_skb = skb_rb_next(skb);
-	int skb_size, next_skb_size;
+	int next_skb_size;
 
-	skb_size = skb->len;
 	next_skb_size = next_skb->len;
 
 	BUG_ON(tcp_skb_pcount(skb) != 1 || tcp_skb_pcount(next_skb) != 1);