Remove documentation of wxXmlDocument::GetEncoding()

This function was removed in 4519d8e08a (Remove wxUSE_UNICODE checks as
they're always true now, 2022-10-27).

interface/wx/xml/xml.h

@@ -760,14 +760,6 @@ public:
     */
     wxXmlNode* DetachRoot();
 
-    /**
-        Returns encoding of in-memory representation of the document
-        (same as passed to Load() or constructor, defaults to UTF-8).
-
-        @note this is meaningless in Unicode build where data are stored as @c wchar_t*.
-    */
-    wxString GetEncoding() const;
-
     /**
         Returns encoding of document (may be empty).