[TCP]: tcp_rcv_rtt_measure_ts() call in pure-ACK path is superfluous

We only want to take receive RTT mesaurements for data
bearing frames, here in the header prediction fast path
for a pure-sender, we know that we have a pure-ACK and
thus the checks in tcp_rcv_rtt_mesaure_ts() will not pass.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -3931,8 +3931,6 @@ int tcp_rcv_established(struct sock *sk, struct sk_buff *skb,
 				    tp->rcv_nxt == tp->rcv_wup)
 					tcp_store_ts_recent(tp);
 
-				tcp_rcv_rtt_measure_ts(sk, skb);
-
 				/* We know that such packets are checksummed
 				 * on entry.
 				 */