code: def _remove_io_handler(self, handler):
          
          if handler in self._unprepared_handlers:
              old_fileno = self._unprepared_handlers[handler]
              del self._unprepared_handlers[handler]
          else:
              old_fileno = handler.fileno()
          if old_fileno is not None:
              del self._handlers[old_fileno]
              self.io_loop.remove_handler(handler.fileno())
docs: Remove an i/o-handler.