Remove QualityScaler framerate reduction.

Framerate-reduction code is disabled on all platforms, and this code
adds complexity. It's necessary to react fast, especially on mobile
platforms or other bad network conditions and framerate reduction adds
another step between HD and QVGA.

BUG=webrtc:5678, webrtc:5830
R=jackychen@webrtc.org, mflodman@webrtc.org

Review URL: https://codereview.webrtc.org/1885893002 .

Cr-Commit-Position: refs/heads/master@{#12503}

webrtc/modules/video_processing/frame_preprocessor.h

@@ -46,9 +46,6 @@ class VPMFramePreprocessor {
                               uint32_t height,
                               uint32_t frame_rate);
 
-  // Set target frame rate.
-  void SetTargetFramerate(int frame_rate);
-
   // Update incoming frame rate/dimension.
   void UpdateIncomingframe_rate();