remove useless code of BaseIOStream

Just found a line of duplication in BaseIOStream in iostream.py, [line 206](https://github.com/facebook/tornado/blob/master/tornado/iostream.py#L206).

It  re-assigns value to `self._stream_callback`, which is already done in [line 198](https://github.com/facebook/tornado/blob/master/tornado/iostream.py#L198).