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/include/video_processing.h

@@ -79,8 +79,6 @@ class VideoProcessing {
                                       uint32_t height,
                                       uint32_t frame_rate) = 0;
 
-  virtual void SetTargetFramerate(int frame_rate) = 0;
-
   virtual uint32_t GetDecimatedFrameRate() = 0;
   virtual uint32_t GetDecimatedWidth() const = 0;
   virtual uint32_t GetDecimatedHeight() const = 0;