cros-md: Remove the non-MD immersive mode code paths

ash/common/material_design/material_design_controller.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/common/material_design/material_design_controller.h"
 
 #include <string>
 
 namespace ash {
 
 // static
 bool MaterialDesignController::IsShelfMaterial() {
   return true;
 }
 
 // static
-bool MaterialDesignController::IsImmersiveModeMaterial() {
-  return true;
-}
-
-// static
 bool MaterialDesignController::IsSystemTrayMenuMaterial() {
   return true;
 }
 
 // static
 bool MaterialDesignController::UseMaterialDesignSystemIcons() {
   return true;
 }
 
 }  // namespace ash