Delete VCMEncodedFrame methods Buffer and MutableBuffer

Replaced by inherited method EncodedImage::data().

Bug: webrtc:9378
Change-Id: I4ec75148f578c72ffb407f9cbf6b4232cc9cfcf6
Reviewed-on: https://webrtc-review.googlesource.com/c/116962
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26212}

modules/video_coding/frame_buffer2.cc

@@ -645,10 +645,10 @@ EncodedFrame* FrameBuffer::CombineAndDeleteFrames(
       last_frame->video_timing().receive_finish_ms;
 
   // Append all remaining frames to the first one.
-  uint8_t* buffer = first_frame->MutableBuffer() + first_frame->size();
+  uint8_t* buffer = first_frame->data() + first_frame->size();
   for (size_t i = 1; i < frames.size(); ++i) {
     EncodedFrame* next_frame = frames[i];
-    memcpy(buffer, next_frame->Buffer(), next_frame->size());
+    memcpy(buffer, next_frame->data(), next_frame->size());
     buffer += next_frame->size();
     delete next_frame;
   }