Remove the unused CipherKeyActivity

chrome/android/java/AndroidManifest.xml

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Copyright 2015 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. -->
 
 <!--
 Note: This is a jinja2 template, processed at build time into the final manifest.
 
 Blocks denoted with { % block some_name % }foo{ % endblock % } can be overridden
 by a child template that "extends" this file.
@@ skipping 522 matching lines @@
         <activity android:name="org.chromium.chrome.browser.media.remote.ExpandedControllerActivity"
             android:theme="@style/MainTheme"
             android:label="Chrome.ExpandedControllerActivity"
             android:hardwareAccelerated="true"
             android:launchMode="singleTask"
             android:noHistory="true"
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|mcc|mnc|screenLayout|smallestScreenSize"
             android:excludeFromRecents="true">
         </activity>
 
-        <!-- This activity is started along with IncognitoDocumentActivity, so it also must
-             disable hardware acceleration. Besides, it doesn't have any UI. -->
-        <activity android:name="org.chromium.chrome.browser.document.CipherKeyActivity"
-            android:launchMode="singleInstance"
-            android:theme="@android:style/Theme.Translucent.NoTitleBar"
-            android:excludeFromRecents="true"
-            android:hardwareAccelerated="false">
-        </activity>
-
         <!-- This activity is used to restart the main Chrome process.  Should never be exported. -->
         <activity android:name="org.chromium.chrome.browser.BrowserRestartActivity"
             android:launchMode="singleInstance"
             android:exported="false"
             android:theme="@android:style/Theme.Translucent.NoTitleBar"
             android:excludeFromRecents="true"
             android:process=":browser_restart_process">
         </activity>
 
         <!-- This activity is to expose the print option via the generic Android share action. -->
@@ skipping 364 matching lines @@
              choose the first MediaRouteController that can play it, so the order of the list can be important.
              The most specific MediaRouteControllers should be listed first, followed by more generic ones.
              The downstream manifest replaces this block, and hence replaces the list of media route
              controllers with its own list. -->
         <meta-data android:name="org.chromium.content.browser.REMOTE_MEDIA_PLAYERS"
             android:value="org.chromium.chrome.browser.media.remote.DefaultMediaRouteController"/>
 
         {% endblock %}
     </application>
 </manifest>