[TCP] Highspeed: Limited slow-start is nowadays in tcp_slow_start

Reuse limited slow-start (RFC3742) included into tcp_cong instead
of having another implementation in High Speed TCP.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>