Make GetBitstream non-virtual since it is no longer needed for testing.

Bug: webrtc:10979
Change-Id: Id313c7fddbec40b9f19dae95f736379b872e3082
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/153663
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29242}

modules/video_coding/rtp_frame_reference_finder_unittest.cc

@@ -40,11 +40,6 @@ class FakePacketBuffer : public PacketBuffer {
     return true;
   }
 
-  bool GetBitstream(const RtpFrameObject& frame,
-                    uint8_t* destination) override {
-    return true;
-  }
-
  private:
   std::map<uint16_t, VCMPacket> packets_;
 };