xdg-shell: remove constraint on popup parents

It seems that this was partially done in
a3cf97ff982638bf7ed23b4303eba280c521b54d; this patch just corrects an
oversight.

Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
diff --git a/stable/xdg-shell/xdg-shell.xml b/stable/xdg-shell/xdg-shell.xml
index ad96fe8..25706e6 100644
--- a/stable/xdg-shell/xdg-shell.xml
+++ b/stable/xdg-shell/xdg-shell.xml
@@ -1031,9 +1031,6 @@
       surface of their own is clicked should dismiss the popup using the destroy
       request.
 
-      The parent surface must have either the xdg_toplevel or xdg_popup surface
-      role.
-
       A newly created xdg_popup will be stacked on top of all previously created
       xdg_popup surfaces associated with the same xdg_toplevel.