Mus: Delete unused Mandoline Surfaces code

components/mus/ws/test_server_window_delegate.cc

 // Copyright 2014 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 "components/mus/surfaces/surfaces_state.h"
 #include "components/mus/ws/test_server_window_delegate.h"
 #include "components/mus/ws/server_window.h"
 
 namespace mus {
 
@@ skipping 11 matching lines @@
 void TestServerWindowDelegate::OnScheduleWindowPaint(ServerWindow* window) {}
 
 const ServerWindow* TestServerWindowDelegate::GetRootWindow(
     const ServerWindow* window) const {
   return root_window_;
 }
 
 void TestServerWindowDelegate::ScheduleSurfaceDestruction(
     ServerWindow* window) {}
 
-ServerWindow* TestServerWindowDelegate::FindWindowForSurface(
-    const ServerWindow* ancestor,
-    mojom::SurfaceType surface_type,
-    const ClientWindowId& client_window_id) {
-  return nullptr;
-}
-
 }  // namespace ws
 
 }  // namespace mus