Makes AshTestImpl::Create create the right impl based on config

ash/mus/test/ash_test_impl_mus.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/mus/test/ash_test_impl_mus.h"
 
 #include "ash/common/test/ash_test.h"
 #include "ash/common/wm_window.h"
 #include "base/memory/ptr_util.h"
 #include "services/ui/public/cpp/property_type_converters.h"
@@ skipping 79 matching lines @@
           window->GetDisplayNearestWindow().id());
 }
 
 void AshTestImplMus::AddTransientChild(WmWindow* parent, WmWindow* window) {
   // TODO(sky): remove this as both classes can share same implementation now.
   ::wm::AddTransientChild(WmWindow::GetAuraWindow(parent),
                           WmWindow::GetAuraWindow(window));
 }
 
 }  // namespace mus
-
-// static
-std::unique_ptr<AshTestImpl> AshTestImpl::Create() {
-  return base::MakeUnique<mus::AshTestImplMus>();
-}
-
 }  // namespace ash