Remove source file writer from VideoProcessor.

It serves a very limited purpose: converting from the input YUV
file to an output Y4M file. The experimenter can do this manually,
if this is of interest. (It is generally not.)

BUG=webrtc:6634

Review-Url: https://codereview.webrtc.org/2993063002
Cr-Commit-Position: refs/heads/master@{#19257}

webrtc/modules/video_coding/codecs/tools/video_quality_measurement.cc

@@ -508,8 +508,8 @@ int main(int argc, char* argv[]) {
   }
   webrtc::test::VideoProcessor* processor = new webrtc::test::VideoProcessor(
       encoder, decoder, &frame_reader, &frame_writer, &packet_manipulator,
-      config, &stats, nullptr /* source_frame_writer */,
-      nullptr /* encoded_frame_writer */, nullptr /* decoded_frame_writer */);
+      config, &stats, nullptr /* encoded_frame_writer */,
+      nullptr /* decoded_frame_writer */);
   processor->Init();
 
   int frame_number = 0;