Cleanup mojo Wallpaper interfaces for mash.

ash/default_wallpaper_delegate.cc

Index: ash/default_wallpaper_delegate.cc
diff --git a/ash/default_wallpaper_delegate.cc b/ash/default_wallpaper_delegate.cc
index 8de44717983aeae2ccb47c5c983340a1a1d14dde..1e78a74a786948c013b9ad56a10fcc4ed79027f9 100644
--- a/ash/default_wallpaper_delegate.cc
+++ b/ash/default_wallpaper_delegate.cc
@@ -31,8 +31,6 @@ void DefaultWallpaperDelegate::InitializeWallpaper() {
   WmShell::Get()->wallpaper_controller()->CreateEmptyWallpaper();
 }
 
-void DefaultWallpaperDelegate::OpenSetWallpaperPage() {}
-
 bool DefaultWallpaperDelegate::CanOpenSetWallpaperPage() {
   return false;
 }