Remove SurfaceSequence from mus client code.

ui/aura/mus/window_port_mus.cc

Index: ui/aura/mus/window_port_mus.cc
diff --git a/ui/aura/mus/window_port_mus.cc b/ui/aura/mus/window_port_mus.cc
index d3862ac13a7205320930ddb3ae7b5dd46de84826..fd997019c1c13f0e783cdc911317af1110263033 100644
--- a/ui/aura/mus/window_port_mus.cc
+++ b/ui/aura/mus/window_port_mus.cc
@@ -222,9 +222,7 @@ void WindowPortMus::SetSurfaceIdFromServer(
         surface_info ? surface_info->surface_id : cc::SurfaceId();
     if (!existing_surface_id.is_null() &&
         existing_surface_id != new_surface_id) {
-      // Return the existing surface sequence.
-      window_tree_client_->OnWindowMusSurfaceDetached(
-          this, surface_info_->surface_sequence);
+      // TODO(kylechar): Start return reference here?
     }
   }
   WindowPortMus* parent = Get(window_->parent());