Mus: Delete unused Mandoline Surfaces code

components/mus/ws/test_server_window_delegate.cc

Index: components/mus/ws/test_server_window_delegate.cc
diff --git a/components/mus/ws/test_server_window_delegate.cc b/components/mus/ws/test_server_window_delegate.cc
index 1660e7826970a297112cb383d5db17c379aba64b..57f1aaec6edfa3f8cbab785d790186cd9e61f175 100644
--- a/components/mus/ws/test_server_window_delegate.cc
+++ b/components/mus/ws/test_server_window_delegate.cc
@@ -29,13 +29,6 @@ const ServerWindow* TestServerWindowDelegate::GetRootWindow(
 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