histograms: Mark Crash.Sender.*Runtime obsolete.

We've made the relevant decision on a timeout and no longer need these
metrics.

Code sending this metric was removed in https://crrev.com/c/2605214.

BUG=1156196
TEST=CQ

Fixed: 1156196
Change-Id: I28fe2abb37980d22fd351e8ed55f68f074b9d3ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2606369Reviewed-by: Ian Barkley-Yeung <iby@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Miriam Zimmerman <mutexlox@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839636}

tools/metrics/histograms/histograms_xml/others/histograms.xml

@@ -3298,6 +3298,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="CrashReport.Sender.ActiveRuntime" units="ms"
     expires_after="2021-05-23">
+  <obsolete>
+    As of 2020-12, we've decided on a timeout and no longer need this metric.
+  </obsolete>
   <owner>mutexlox@chromium.org</owner>
   <owner>iby@chromium.org</owner>
   <owner>cros-telemetry@google.com</owner>
@@ -3310,6 +3313,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="CrashReport.Sender.Runtime" units="ms"
     expires_after="2021-01-15">
+  <obsolete>
+    As of 2020-12, we've decided on a timeout and no longer need this metric.
+  </obsolete>
   <owner>mutexlox@chromium.org</owner>
   <owner>iby@chromium.org</owner>
   <owner>cros-telemetry@google.com</owner>