Obsolete unused crash reporter metrics

One of these already expired and the other is going to soon. These are
currently unused by anybody so they are being marked obsolete and will
be removed from client side code in a Chrome OS CL.

CL which removes them from ChromeOS: crrev.com/c/1763923

Bug: 975639
Test: None
Change-Id: Id7da189f57b26ba9140a899f304f4dfb360c49f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1763632
Commit-Queue: Jeffrey Kardatzke <jkardatzke@google.com>
Auto-Submit: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689559}

tools/metrics/histograms/histograms.xml

@@ -101603,6 +101603,9 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Platform.KernelWarningHashes" expires_after="2018-08-30">
+  <obsolete>
+    Removed 8/2019 due to lack of usage.
+  </obsolete>
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>
     The 32-bit hash of a kernel warning. This is the hash of the
@@ -101844,6 +101847,9 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Platform.ServiceFailureHashes" expires_after="M77">
+  <obsolete>
+    Removed 8/2019 due to lack of usage.
+  </obsolete>
   <owner>ddavenport@chromium.org</owner>
   <summary>
     The 32-bit hash of the name of the service that failed. The name of the