network: Drop Mac80211Monitor metrics

These are related to a module that's getting dropped from Shill in
https://crrev.com/c/2590497.

Bug: b:172219309
Change-Id: If758f1f239b1ab8967d1dc6e6c16666fc0439523
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2590747Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Brian Norris <briannorris@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837173}

tools/metrics/histograms/histograms_xml/network/histograms.xml

@@ -1602,6 +1602,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Network.Shill.WiFi.StoppedTxQueueLength" units="frames"
     expires_after="2021-12-01">
+  <obsolete>
+    Removed 12/2020
+  </obsolete>
   <owner>briannorris@chromium.org</owner>
   <owner>cros-network-metrics@google.com</owner>
   <summary>
@@ -1614,6 +1617,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Network.Shill.WiFi.StoppedTxQueueReason"
     enum="NetworkQueueStopReason" expires_after="2021-12-01">
+  <obsolete>
+    Removed 12/2020
+  </obsolete>
   <owner>briannorris@chromium.org</owner>
   <owner>cros-network-metrics@google.com</owner>
   <summary>