Delete method EncodedFrame::GetBitstream, part 1

Only caller was the RtpFrameObject constructor, so it's
not needed in the interface.

To be able to delete downstream overrides, add a temporary
default implementation. Method will be completely deleted in part 2.

Bug: webrtc:9378
Change-Id: I9083b6284313b6ebce854c6f2cec4617953331d9
Reviewed-on: https://webrtc-review.googlesource.com/c/112128
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25822}

modules/video_coding/frame_buffer2_unittest.cc

@@ -88,8 +88,6 @@ class VCMJitterEstimatorMock : public VCMJitterEstimator {
 
 class FrameObjectFake : public EncodedFrame {
  public:
-  bool GetBitstream(uint8_t* destination) const override { return true; }
-
   int64_t ReceivedTime() const override { return 0; }
 
   int64_t RenderTime() const override { return _renderTimeMs; }