Nothing here, including the event table for this class using
non-existing wxBitmapButton::OnSysColourChanged (which is just the same
as wxWindow::OnSysColourChanged, in fact), was needed any longer ever
since the changes of a6fd73d33a (implement wxBitmapButton as just a
wrapper for wxButton under MSW, 2009-06-15), so simply remove all the
unused stuff.