Deprecate Fingerprint.Unlock.MigrationForPositiveMatchSecretResult

The metric is removed in http://crrev/c/2438770. Mark it as obsolete.

Bug: b:169082568
Change-Id: I844790fd1139aa101726dfce8e28491fffe140ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438878
Commit-Queue: Yicheng Li <yichengli@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Auto-Submit: Yicheng Li <yichengli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812506}

tools/metrics/histograms/histograms_xml/fingerprint/histograms.xml

@@ -124,6 +124,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Fingerprint.Unlock.MigrationForPositiveMatchSecretResult"
     enum="BooleanSuccess" expires_after="M85">
+  <obsolete>
+    Removed 09/2020 as the migration is no longer supported after M85.
+  </obsolete>
   <owner>yichengli@chromium.org</owner>
   <owner>chromeos-fingerprint@google.com</owner>
   <summary>