Remove differ from ScreenCapturer implementations

webrtc/modules/desktop_capture/desktop_capture.gypi

 # Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 {
   'targets': [
@@ skipping 122 matching lines @@
         }],
         ['OS!="win" and OS!="mac" and use_x11==0', {
           'sources': [
             'mouse_cursor_monitor_null.cc',
             'screen_capturer_null.cc',
             'window_capturer_null.cc',
           ],
         }],
         ['OS!="ios" ', {
           'sources': [
-            'differ.cc',
-            'differ.h',
             'differ_block.cc',
             'differ_block.h',
             'screen_capturer_differ_wrapper.cc',
             'screen_capturer_differ_wrapper.h',
           ],
         }],
         ['OS=="mac"', {
           'link_settings': {
             'libraries': [
               '$(SDKROOT)/System/Library/Frameworks/AppKit.framework',
@@ skipping 37 matching lines @@
               'xcode_settings': {
                 'OTHER_CFLAGS': [ '-msse2', ],
               },
             }],
           ],
         },
       ],  # targets
     }],
   ],
 }