This reverts commit c657fd3d61 because
changing the format of DDE advise requests/replies is not a good idea:
other applications (those using previous versions of wxWidgets or even
not using wxWidgets at all) may rely on getting data in real CF_TEXT
format rather than in one of text formats preceded by the extra byte
containing the actual format and the previous commit would have silently
broken this.
Another fix for #17900 will be implemented instead.
09bf235a59