Bug in cubic.rs

[WernerDrasche]

I think that the following code passage can be removed from cubic.rs as it causes unnecessary packet loss and tanks performance when combined with qdisc pacing.

// Detecting spurious congestion events.
// <https://tools.ietf.org/id/draft-ietf-tcpm-rfc8312bis-00.html#section-4.9>
//
// When the recovery episode ends with recovering
// a few packets (less than cwnd / mss * ROLLBACK_THRESHOLD_PERCENT(%)), it's
// considered as spurious and restore to the previous state.
if r.congestion_recovery_start_time.is_some() {
  let new_lost = r.lost_count - r.cubic_state.prior.lost_count;

  let rollback_threshold = (r.congestion_window / r.max_datagram_size) *
      ROLLBACK_THRESHOLD_PERCENT /
      100;

  let rollback_threshold = rollback_threshold.max(MIN_ROLLBACK_THRESHOLD);

    if new_lost < rollback_threshold {
        let did_rollback = rollback(r);
        if did_rollback {
            return;
        }
    }
}