Revert of Injectable Obj-C video codecs (patchset #8 id:140001 of https://codereview.webrtc.org/2966023002/ )

Reason for revert:
Causes no video in certain scenarios. Please come up with a test plan or unit test to prevent such problems in the future.

Original issue's description:
> Injectable Obj-C video codecs
>
> Initial CL for this effort, with a working RTCVideoEncoder/Decoder for H264
> (wrapping the VideoToolbox codec).
>
> Some notes / things left to do:
>   - There are some hard-coded references to codec types that are supported by
>     webrtc::VideoCodec, cricket::VideoCodec, webrtc::CodecSpecificInfo etc
>     since we need to convert to/from these types in ObjCVideoEncoder/Decoder.
>     These types would need to be more codec agnostic to avoid this.
>   - Most interfaces are borrowed from the design document for injectable
>     codecs in Android. Some data in the corresponding C++ classes is discarded
>     when converting to the Obj-C version, since it has fewer fields. I have not
>     verified whether all data that we do keep is needed, or whether we might be
>     losing anything useful in these conversions.
>   - Implement the VideoToolbox codec code directly in the RTCVideoEncoderH264
>     classes, instead of wrapping webrtc::H264VideoToolboxEncoder / decoder.
>     Eliminates converting between ObjC/C++ types outside the ObjCVideoEncoder/
>     Decoder wrapper classes.
>   - List the injected codec factory's supported codecs in the list of codecs in
>     AppRTCMobile.
>
> BUG=webrtc:7924
> R=magjed@webrtc.org
>
> Review-Url: https://codereview.webrtc.org/2966023002 .
> Cr-Commit-Position: refs/heads/master@{#18928}
> Committed: a0349c138d

TBR=magjed@webrtc.org,andersc@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:7924
NOTRY=true

Review-Url: https://codereview.webrtc.org/2975963002
Cr-Commit-Position: refs/heads/master@{#18979}

webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCVideoFrame.mm

@@ -17,7 +17,6 @@
 }
 
 @synthesize buffer = _buffer;
-@synthesize timeStamp;
 
 - (int)width {
   return _buffer.width;