message: MetalBackend.h: Remove dead declarations
diff: namespace dawn_native { namespace metal {
  DAWN_NATIVE_EXPORT dawnDevice CreateDevice(id<MTLDevice> metalDevice);
  - DAWN_NATIVE_EXPORT void SetNextDrawable(dawnDevice device, id<CAMetalDrawable> drawable);
  - DAWN_NATIVE_EXPORT void Present(dawnDevice device);
  }} // namespace dawn_native::metal
  #endif // DAWNNATIVE_METALBACKEND_H_
  