tcp: remove obsolete comment about TCP_SKB_CB(skb)->when in tcp_fragment()

The TCP_SKB_CB(skb)->when field no longer exists as of recent change
7faee5c0d514 ("tcp: remove TCP_SKB_CB(skb)->when"). And in any case,
tcp_fragment() is called on already-transmitted packets from the
__tcp_retransmit_skb() call site, so copying timestamps of any kind
in this spot is quite sensible.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Reported-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -1146,9 +1146,6 @@ int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len,
 
 	buff->ip_summed = skb->ip_summed;
 
-	/* Looks stupid, but our code really uses when of
-	 * skbs, which it never sent before. --ANK
-	 */
 	buff->tstamp = skb->tstamp;
 	tcp_fragment_tstamp(skb, buff);