Delete left-over forward declaration of RTPFragmentationHeader.

Was overlooked in cl https://webrtc-review.googlesource.com/75180.

Bug: webrtc:6471
Change-Id: I0abc26b6c77096d6674a6fe487cbb2d94269eb96
Reviewed-on: https://webrtc-review.googlesource.com/78261
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23345}
diff --git a/api/video_codecs/video_decoder.h b/api/video_codecs/video_decoder.h
index c435094..87f608a 100644
--- a/api/video_codecs/video_decoder.h
+++ b/api/video_codecs/video_decoder.h
@@ -21,7 +21,6 @@
 
 namespace webrtc {
 
-class RTPFragmentationHeader;
 // TODO(pbos): Expose these through a public (root) header or change these APIs.
 struct CodecSpecificInfo;
 class VideoCodec;