Fix signal disconnect in thunar_window_unrealize

Commit c2a97b89 replaced the gtk_widget_queue_draw callback with
thunar_window_clipboard_manager_changed. The same change needs to be
made in thunar_window_unrealize, which was still referencing the old
function (gtk_widget_queue_draw).

Fixes: c2a97b89 ("Fix file-icon update when file is cut (Issue #588)")
Closes: xfce/thunar#653