[ash-md] Remove non-md code from the Shelf background animations.

ash/common/shelf/shelf_background_animator_observer.h

Index: ash/common/shelf/shelf_background_animator_observer.h
diff --git a/ash/common/shelf/shelf_background_animator_observer.h b/ash/common/shelf/shelf_background_animator_observer.h
index b90e8814ed1443773aaf6033438962c25fd299f6..a90b30867fc020bc15dbfbf5297cc4df990fa35c 100644
--- a/ash/common/shelf/shelf_background_animator_observer.h
+++ b/ash/common/shelf/shelf_background_animator_observer.h
@@ -15,10 +15,6 @@ class ASH_EXPORT ShelfBackgroundAnimatorObserver {
   // Called when the Shelf's opaque background should be updated.
   virtual void UpdateShelfOpaqueBackground(int alpha) {}
 
-  // Called when the Shelf's asset based background should be updated.
-  // TODO(bruthig): Remove when non-md is no longer needed (crbug.com/614453).
-  virtual void UpdateShelfAssetBackground(int alpha) {}
-
   // Called when the Shelf item (aka button) backgrounds should be updated.
   virtual void UpdateShelfItemBackground(int alpha) {}