Revert "Add spatial index to EncodedImage."

This reverts commit da0898dfae3b0a013ca8ad3828e9adfdc749748d.

Reason for revert: Broke downstream tests.

Original change's description:
> Add spatial index to EncodedImage.
> 
> Replaces the VP8 simulcast index and VP9 spatial index formely part of
> CodecSpecificInfo.
> 
> Bug: webrtc:9378
> Change-Id: I80eafd63fbdee0a25864338196a690628b4bd3d2
> Reviewed-on: https://webrtc-review.googlesource.com/83161
> Commit-Queue: Niels Moller <nisse@webrtc.org>
> Reviewed-by: Erik Språng <sprang@webrtc.org>
> Reviewed-by: Sebastian Jansson <srte@webrtc.org>
> Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
> Reviewed-by: Philip Eliasson <philipel@webrtc.org>
> Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#24485}

TBR=brandtr@webrtc.org,magjed@webrtc.org,nisse@webrtc.org,sprang@webrtc.org,philipel@webrtc.org,srte@webrtc.org

Change-Id: Idb4fb9d72e5574d7353c631cb404a1311f3fd148
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9378
Reviewed-on: https://webrtc-review.googlesource.com/96664
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24486}

modules/video_coding/codecs/vp9/vp9_impl.h

@@ -61,7 +61,6 @@ class VP9EncoderImpl : public VP9Encoder {
   int InitAndSetControlSettings(const VideoCodec* inst);
 
   void PopulateCodecSpecific(CodecSpecificInfo* codec_specific,
-                             absl::optional<int>* spatial_idx,
                              const vpx_codec_cx_pkt& pkt,
                              uint32_t timestamp,
                              bool first_frame_in_picture);