Revert "Fix echo return loss stats and add to RTCAudioSourceStats."

This reverts commit a27cfbffdfa0bf359628d2164db5b9d6321f9c9c.

Reason for revert: WebRtcBrowserTest.RunsAudioVideoWebRTCCallInTwoTabsGetStatsPromise failing.

Original change's description:
> Fix echo return loss stats and add to RTCAudioSourceStats.
>
> This solves two problems:
> * Echo return loss stats weren't being gathered in Chrome, because they
>   need to be taken from the audio processor attached to the track
>   rather than the audio send stream.
> * The standardized location is in RTCAudioSourceStats, not
>   RTCMediaStreamTrackStats. For now, will populate the stats in both
>   locations.
>
> Bug: webrtc:12770
> Change-Id: I47eaf7f2b50b914a1be84156aa831e27497d07e3
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/223182
> Commit-Queue: Taylor Brandstetter <deadbeef@webrtc.org>
> Reviewed-by: Henrik Boström <hbos@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#34344}

TBR=deadbeef@webrtc.org,hbos@webrtc.org,hbos@chromium.org,webrtc-scoped@luci-project-accounts.iam.gserviceaccount.com

Change-Id: I6b2587d762f005adef67c0d5121f1b58c3b76688
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:12770
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/223068
Reviewed-by: Evan Shrubsole <eshr@google.com>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Evan Shrubsole <eshr@google.com>
Cr-Commit-Position: refs/heads/master@{#34352}

api/stats/rtcstats_objects.h

@@ -629,8 +629,6 @@ class RTC_EXPORT RTCAudioSourceStats final : public RTCMediaSourceStats {
   RTCStatsMember<double> audio_level;
   RTCStatsMember<double> total_audio_energy;
   RTCStatsMember<double> total_samples_duration;
-  RTCStatsMember<double> echo_return_loss;
-  RTCStatsMember<double> echo_return_loss_enhancement;
 };
 
 // https://w3c.github.io/webrtc-stats/#dom-rtcvideosourcestats