window: Don't let solid-csd linger

Just from reading the code, it seems that we
should unset .csd and .solid-csd at the same
time, since the are mutually exclusive and
we unset them here so realize() can set one
of them again.

gtk/gtkwindow.c

@@ -3038,8 +3038,10 @@ gtk_window_set_titlebar (GtkWindow *window,
 
   if (titlebar == NULL)
     {
+      /* these are updated in realize() */
       priv->client_decorated = FALSE;
       gtk_widget_remove_css_class (widget, "csd");
+      gtk_widget_remove_css_class (widget, "solid-csd");
 
       goto out;
     }