[TCP] Vegas: Remove extra call to tcp_vegas_rtt_calc

Remove unneeded call to tcp_vegas_rtt_calc. The more accurate
microsecond value has already been registered prior to calling
tcp_vegas_cong_avoid.

Signed-off-by: Thomas Young <tyo@ee.mu.oz.au>
Signed-off-by: David S. Miller <davem@davemloft.net>