Revert of Add content type information to encoded images and corresponding rtp extension header (patchset #31 id:600001 of https://codereview.webrtc.org/2772033002/ )

Reason for revert:
Breaks dependent projects.

Original issue's description:
> Add content type information to Encoded Images and add corresponding RTP extension header.
> Use it to separate UMA e2e delay metric between screenshare from video.
> Content type extension is set based on encoder settings and processed and decoders.
>
> Also,
> Fix full-stack-tests to calculate RTT correctly, so new metric could be tested.
>
> BUG=webrtc:7420
>
> Review-Url: https://codereview.webrtc.org/2772033002
> Cr-Commit-Position: refs/heads/master@{#17640}
> Committed: 64e739aeae

TBR=tommi@webrtc.org,sprang@webrtc.org,stefan@webrtc.org,nisse@webrtc.org,mflodman@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7420

Review-Url: https://codereview.webrtc.org/2816463002
Cr-Commit-Position: refs/heads/master@{#17644}

webrtc/modules/video_coding/generic_decoder.h

@@ -30,7 +30,6 @@ struct VCMFrameInformation {
   int64_t decodeStartTimeMs;
   void* userData;
   VideoRotation rotation;
-  VideoContentType content_type;
 };
 
 class VCMDecodedFrameCallback : public DecodedImageCallback {
@@ -110,7 +109,6 @@ class VCMGenericDecoder {
   VideoCodecType _codecType;
   bool _isExternal;
   bool _keyFrameDecoded;
-  VideoContentType _last_keyframe_content_type;
 };
 
 }  // namespace webrtc