Fix wxFileDialog::SetExtraControlCreator() supported platforms

Just remove them, actually, as this functionality is supported by all 3
major platforms.

interface/wx/filedlg.h

@@ -296,8 +296,6 @@ public:
         The @c creator function should take pointer to parent window (file dialog)
         and should return a window allocated with operator new.
 
-        Supported platforms: wxGTK, wxMSW, wxUniv.
-
         @since 2.9.0
     */
     bool SetExtraControlCreator(ExtraControlCreatorFunction creator);