message: Remove stale function body
diff: @@ -87,10 +87,6 @@ optional<message> behavior_impl::invoke(message& xs) {
  return none;
  }
  -bool behavior_impl::invoke(detail::invoke_result_visitor& f, message& xs) {
  - return invoke(f, xs);
  -}
  -
  void behavior_impl::handle_timeout() {
  // nop
  }
  