[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: David S. Miller <davem@davemloft.net>