Remove document mode opt-out promo.

tools/metrics/actions/actions.xml

 <!--
 Copyright 2014 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is a comprehensive list of Chrome user actions along with the owner
 and description for each user action. A user action should be tied to an actual
 action taken by a user. It should be logged immediately after a user takes the
@@ skipping 2887 matching lines @@
 </action>
 
 <action name="DockingWindow_Top">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <description>Please enter the description of this user action.</description>
 </action>
 
 <action name="DocumentActivity_OptOutShownOnHome">
   <owner>mariakhomenko@chromium.org</owner>
   <description>User was presented with the opt out promo.</description>
+  <obsolete>Removed in M51.</obsolete>
 </action>
 
 <action name="DocumentActivity_UserOptIn">
   <owner>mariakhomenko@chromium.org</owner>
   <description>User opted in to document mode through settings.</description>
+  <obsolete>Removed in M51.</obsolete>
 </action>
 
 <action name="DocumentActivity_UserOptOut">
   <owner>mariakhomenko@chromium.org</owner>
   <description>User opted out of document mode through settings.</description>
+  <obsolete>Removed in M51.</obsolete>
 </action>
 
 <action name="DomDistiller_DistilledPageOpened">
   <owner>smaslo@chromium.org</owner>
   <owner>yfriedman@chromium.org</owner>
   <description>User opens reader mode on a page.</description>
 </action>
 
 <action name="DomDistiller_DistilledPagePrefsOpened">
   <owner>smaslo@chromium.org</owner>
@@ skipping 11116 matching lines @@
 <action name="webapps.AddShortcut.Manifest">
   <owner>mlamouri@chromium.org</owner>
   <description>
     Shortcut added to the homescreen with a valid Manifest. This action will be
     recorded in addition to AppShortcut, AppShortcutApple and Bookmark actions
     to keep a trend of whether the webapp also add meta tags.
   </description>
 </action>
 
 </actions>