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.

include/wx/msw/dde.h

@@ -70,6 +70,7 @@ public:
   WXHCONV       m_hConv;
   const void*   m_sendingData;
   int           m_dataSize;
+  wxIPCFormat   m_dataType;
 
   wxDECLARE_NO_COPY_CLASS(wxDDEConnection);
   wxDECLARE_DYNAMIC_CLASS(wxDDEConnection);