Mark UMA WebRTC.Audio.ResidualEchoDetector.EchoLikelihood obsolete

The metric is no longer reported as of M84, http://crrev.com/767709.

Change-Id: Ib050a9c437856a680e6c37c5fc15c4616345a24c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2624732
Auto-Submit: Henrik Lundin <hlundin@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842776}

tools/metrics/histograms/histograms_xml/web_rtc/histograms.xml

@@ -772,6 +772,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="WebRTC.Audio.ResidualEchoDetector.EchoLikelihood" units="%"
     expires_after="2021-05-09">
+  <obsolete>
+    No longer reported. Removed in M84.
+  </obsolete>
   <owner>hlundin@chromium.org</owner>
   <owner>ivoc@chromium.org</owner>
   <summary>