Remove support for the keygen tag

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.
 
@@ skipping 11302 matching lines @@
   <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="Options_DefaultKeygenSettingChanged">
   <owner>rsleevi@chromium.org</owner>
   <owner>svaldez@chromium.org</owner>
   <description>
     Triggers when the default decision for using key generation is changed.
   </description>
+  <obsolete>Support for Keygen was removed.</obsolete>
 </action>
 
 <action name="Options_DefaultMIDISysExSettingChanged">
   <owner>toyoshim@chromium.org</owner>
   <description>
     Triggers when the default decision for Midi SysEx permissions is changed.
   </description>
 </action>
 
 <action name="Options_DefaultMediaStreamCameraSettingChanged">
@@ skipping 4775 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>