mash: Refactor ChromeLauncherController's ShelfDelegate code.

ash/mus/shelf_delegate_mus.cc

Index: ash/mus/shelf_delegate_mus.cc
diff --git a/ash/mus/shelf_delegate_mus.cc b/ash/mus/shelf_delegate_mus.cc
index 90dd0f91dc864d5ad9f23d6921b52ca531387a90..8136fcc461384f7a61be343c28ef62cf95fb30c3 100644
--- a/ash/mus/shelf_delegate_mus.cc
+++ b/ash/mus/shelf_delegate_mus.cc
@@ -29,10 +29,6 @@ ShelfID ShelfDelegateMus::GetShelfIDForAppIDAndLaunchID(
   return ShelfDelegateMus::GetShelfIDForAppID(app_id);
 }
 
-bool ShelfDelegateMus::HasShelfIDToAppIDMapping(ShelfID id) const {
-  return Shell::Get()->shelf_controller()->shelf_id_to_app_id().count(id) > 0;
-}
-
 const std::string& ShelfDelegateMus::GetAppIDForShelfID(ShelfID id) {
   if (Shell::Get()->shelf_controller()->shelf_id_to_app_id().count(id))
     return Shell::Get()->shelf_controller()->shelf_id_to_app_id().at(id);