Cleanup display creation in mus tests.

services/ui/ws/platform_display_default.cc

Index: services/ui/ws/platform_display_default.cc
diff --git a/services/ui/ws/platform_display_default.cc b/services/ui/ws/platform_display_default.cc
index 9492e85cb8fdd0d5da5d16a0032c53b18d6fbefd..c807ae8a71b52e9d2d53f1bf9987148c0fde1b0c 100644
--- a/services/ui/ws/platform_display_default.cc
+++ b/services/ui/ws/platform_display_default.cc
@@ -125,11 +125,6 @@ gfx::Rect PlatformDisplayDefault::GetBounds() const {
   return metrics_.bounds;
 }
 
-bool PlatformDisplayDefault::IsPrimaryDisplay() const {
-  return display::PlatformScreen::GetInstance()->GetPrimaryDisplayId() ==
-         display_id_;
-}
-
 void PlatformDisplayDefault::OnGpuChannelEstablished(
     scoped_refptr<gpu::GpuChannelHost> channel) {
   frame_generator_->OnGpuChannelEstablished(channel);