Clean up video_coding namespace snipets.

Bug: webrtc:12579
Change-Id: I487fe017f30746e2fe83a122123b236295d96d28
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/212962
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33558}

modules/video_coding/frame_object.h

@@ -63,11 +63,6 @@ class RtpFrameObject : public EncodedFrame {
   int times_nacked_;
 };
 
-// TODO(bugs.webrtc.org/12579): Remove when downstream has been updated.
-namespace video_coding {
-using ::webrtc::RtpFrameObject;
-}  // namespace video_coding
-
 }  // namespace webrtc
 
 #endif  // MODULES_VIDEO_CODING_FRAME_OBJECT_H_