Delete dead Device Motion code in Blink.

Source/core/core.gypi

 {
     'variables': {
         'core_idl_files': [
             'css/CSS.idl',
             'css/CSSCharsetRule.idl',
             'css/CSSFontFaceLoadEvent.idl',
             'css/CSSFontFaceRule.idl',
             'css/CSSHostRule.idl',
             'css/CSSImportRule.idl',
             'css/CSSMediaRule.idl',
@@ skipping 3046 matching lines @@
             'platform/mediastream/RTCDataChannelHandlerClient.h',
             'platform/mediastream/RTCPeerConnectionHandler.cpp',
             'platform/mediastream/RTCPeerConnectionHandler.h',
             'platform/mediastream/RTCPeerConnectionHandlerClient.h',
             'platform/mediastream/RTCSessionDescriptionRequest.h',
             'platform/mediastream/RTCStatsRequest.h',
             'platform/mediastream/RTCVoidRequest.h',
             'platform/midi/MIDIAccessor.h',
             'platform/midi/MIDIAccessor.cpp',
             'platform/midi/MIDIAccessorClient.h',
-            'platform/mock/DeviceMotionClientMock.cpp',
             'platform/mock/DeviceOrientationClientMock.cpp',
             'platform/mock/GeolocationClientMock.cpp',
             'platform/mock/PlatformSpeechSynthesizerMock.cpp',
             'platform/mock/PlatformSpeechSynthesizerMock.h',
             'platform/mock/ScrollbarThemeMock.cpp',
             'platform/mock/ScrollbarThemeMock.h',
             'platform/network/BlobData.cpp',
             'platform/network/BlobData.h',
             'platform/network/DNS.cpp',
             'platform/network/DNS.h',
@@ skipping 649 matching lines @@
                 'bison_exe': 'bison',
                 # We specify a preprocess so it happens locally and won't get
                 # distributed to goma.
                 # FIXME: /usr/bin/gcc won't exist on OSX forever. We want to
                 # use /usr/bin/clang once we require Xcode 4.x.
                 'preprocessor': '--preprocessor "/usr/bin/gcc -E -P -x c++"'
               }],
          ],
     }
 }