xdg-shell: Remove left-over paragraph from pre positioner versions

It mentioned the now removed x, y parameters of xdg_surface.get_popup.
The xdg_positioner now has the relevant documentation that was
previously documented by the now removed paragraph.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>

stable/xdg-shell/xdg-shell.xml

@@ -1043,12 +1043,6 @@
       The parent of an xdg_popup must be mapped (see the xdg_surface
       description) before the xdg_popup itself.
 
-      The x and y arguments passed when creating the popup object specify
-      where the top left of the popup should be placed, relative to the
-      local surface coordinates of the parent surface. See
-      xdg_surface.get_popup. An xdg_popup must intersect with or be at least
-      partially adjacent to its parent surface.
-
       The client must call wl_surface.commit on the corresponding wl_surface
       for the xdg_popup state to take effect.
     </description>