remove empty unused method

Actually @matthewd noticed that this method is here so we are compliant with the Rack specification (c.f. [here](https://github.com/rack/rack/blob/5a9ffeb77c613e3611262f1182284f1d7df393a8/SPEC#L255)). Even though this line is not reached because of the `X-Cascade`, could you add a comment atop this method so this won't be remove in the future please ?