Don't use a dummy menu with wxHtmlHelpFrame under OS X

Contrary to the comment in wxHtmlHelpFrame::Create(), this doesn't seem to be
necessary and can be actually problematic.

Never adding this menu obviates the need for the wxHF_MENU style from #15538.

Closes #15538.

include/wx/html/helpfrm.h

@@ -126,11 +126,6 @@ protected:
     void OnCloseWindow(wxCloseEvent& event);
     void OnActivate(wxActivateEvent& event);
 
-#ifdef __WXMAC__
-    void OnClose(wxCommandEvent& event);
-    void OnAbout(wxCommandEvent& event);
-#endif
-
     // Images:
     enum {
         IMG_Book = 0,