histograms: Mark Cryptohome.TimeToMountGuestSync as obsolete.

Not reported since 8/2018.

BUG=1088699
TEST=validate_format, pretty_print.

Change-Id: Idc8724b965b84bc99bf222d21555df81eda40e76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2239303
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776995}

tools/metrics/histograms/histograms.xml

@@ -34814,6 +34814,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Cryptohome.TimeToMountGuestSync" units="ms"
     expires_after="M85">
+  <obsolete>
+    Removed 06/2020 as it is not used since crrev.com/c/1183895 (8/2018).
+  </obsolete>
   <owner>apronin@chromium.org</owner>
   <owner>cros-hwsec+uma@chromium.org</owner>
   <summary>