Delete dead Device Motion code in Blink.

Source/modules/modules.gypi

 {
   'variables': {
     # Experimental hooks for embedder to provide extra IDL and source files.
     #
     # Note: this is not a supported API. If you rely on this, you will be broken
     # from time to time as the code generator changes in backward incompatible
     # ways.
     'extra_blink_module_idl_files': [],
     'extra_blink_module_files': [],
 
@@ skipping 202 matching lines @@
       'crypto/NormalizeAlgorithm.cpp',
       'crypto/NormalizeAlgorithm.h',
       'crypto/SubtleCrypto.cpp',
       'crypto/SubtleCrypto.h',
       'crypto/WorkerCrypto.cpp',
       'crypto/WorkerCrypto.h',
       'crypto/WorkerGlobalScopeCrypto.cpp',
       'crypto/WorkerGlobalScopeCrypto.h',
       'device_orientation/DeviceAcceleration.cpp',
       'device_orientation/DeviceAcceleration.h',
-      'device_orientation/DeviceMotionClient.h',
       'device_orientation/DeviceMotionController.cpp',
       'device_orientation/DeviceMotionController.h',
       'device_orientation/DeviceMotionData.cpp',
       'device_orientation/DeviceMotionData.h',
       'device_orientation/DeviceMotionDispatcher.cpp',
       'device_orientation/DeviceMotionDispatcher.h',
       'device_orientation/DeviceMotionEvent.h',
       'device_orientation/DeviceMotionEvent.cpp',
       'device_orientation/DeviceRotationRate.cpp',
       'device_orientation/DeviceRotationRate.h',
@@ skipping 475 matching lines @@
       'websockets/WorkerThreadableWebSocketChannel.h',
     ],
     'modules_unittest_files': [
       'websockets/WebSocketDeflaterTest.cpp',
       'websockets/WebSocketExtensionDispatcherTest.cpp',
       'websockets/WebSocketExtensionParserTest.cpp',
       'websockets/WebSocketPerMessageDeflateTest.cpp',
     ],
   },
 }