Remove support for the keygen tag

chrome/browser/resources/settings/site_settings_page/site_settings_page.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/web_ui_listener_behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button-light.html">
 <link rel="import" href="/icons.html">
 <link rel="import" href="/route.html">
 <link rel="import" href="/settings_shared_css.html">
 <link rel="import" href="/site_settings/constants.html">
 <link rel="import" href="/site_settings/site_settings_behavior.html">
 <link rel="import" href="/site_settings/site_settings_prefs_browser_proxy.html">
 
@@ skipping 108 matching lines @@
         data-route="SITE_SETTINGS_BACKGROUND_SYNC" on-tap="onTapNavigate_"
         actionable>
       <iron-icon icon="settings:sync"></iron-icon>
       <div class="middle">
         $i18n{siteSettingsBackgroundSync}
         <div class="secondary">[[default_.backgroundSync]]</div>
       </div>
       <button class="subpage-arrow" is="paper-icon-button-light"></button>
     </div>
     <div class="settings-box two-line"
-        category$="[[ContentSettingsTypes.KEYGEN]]"
-        data-route="SITE_SETTINGS_KEYGEN" on-tap="onTapNavigate_" actionable>
-      <iron-icon icon="settings:code"></iron-icon>
-      <div class="middle">
-        $i18n{siteSettingsKeygen}
-        <div class="secondary">[[default_.keygen]]</div>
-      </div>
-      <button class="subpage-arrow" is="paper-icon-button-light"></button>
-    </div>
-    <div class="settings-box two-line"
         category$="[[ContentSettingsTypes.AUTOMATIC_DOWNLOADS]]"
         data-route="SITE_SETTINGS_AUTOMATIC_DOWNLOADS"
         on-tap="onTapNavigate_" actionable>
       <iron-icon icon="cr:file-download"></iron-icon>
       <div class="middle">
         $i18n{siteSettingsAutomaticDownloads}
         <div class="secondary">[[default_.multipleAutomaticDownloads]]</div>
       </div>
       <button class="subpage-arrow" is="paper-icon-button-light"></button>
     </div>
@@ skipping 35 matching lines @@
     </div>
     <div class="settings-box" data-route="SITE_SETTINGS_PDF_DOCUMENTS"
         on-tap="onTapNavigate_" actionable>
       <iron-icon icon="settings:pdf"></iron-icon>
       <div class="middle">$i18n{siteSettingsPdfDocuments}</div>
       <button class="subpage-arrow" is="paper-icon-button-light"></button>
     </div>
   </template>
   <script src="site_settings_page.js"></script>
 </dom-module>