Network.Shill.SandboxingEnabled is obsolete

Obsolete for a while:
https://chromium-review.googlesource.com/1516255

Bug: 1089038
Change-Id: Icc8f91001a5d41d92905b24bebbb2aa0ea1a0918
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2236279Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Brian Norris <briannorris@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776639}

tools/metrics/histograms/histograms.xml

@@ -103270,6 +103270,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Network.Shill.SandboxingEnabled" enum="Boolean"
     expires_after="M85">
+  <obsolete>
+    Removed March 2019.
+  </obsolete>
   <owner>mortonm@chromium.org</owner>
   <summary>
     Chrome OS metric signifying whether a system is running shill in a sandbox