Remove QualityScaler kDefaultLowQpDenominator.

This denominator doesn't make any semantic sense, it's better to use
real thresholds for when things look "good" or "bad" rather than
fractions of a max QP.

BUG=webrtc:5678
R=danilchap@webrtc.org

Review URL: https://codereview.webrtc.org/1855393005 .

Cr-Commit-Position: refs/heads/master@{#12363}

webrtc/modules/video_coding/utility/quality_scaler.h

@@ -17,7 +17,6 @@
 namespace webrtc {
 class QualityScaler {
  public:
-  static const int kDefaultLowQpDenominator;
   struct Resolution {
     int width;
     int height;