Revert "Fix passing Unicode strings via wxIPC when using DDE"

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.

docs/changes.txt

@@ -124,10 +124,6 @@ All (GUI):
 - Allow changing tooltip text for button allowing to enter a new string
   in wxPGArrayEditorDialog.
 
-wxMSW:
-
-- Fix passing Unicode strings via wxIPC when using DDE.
-
 
 3.1.2: (released 2018-12-10)
 ----------------------------