This follows up #9869, with an awaited ``next()``, fixing the script.
> I'm looking to remove another 5kb package, replacing it with an in-house script. The commit i'm proposing translates the entire job of koa-remove-trailing-slashes into a 206 byte script that gets the job done well.
(just to show the code does work)

Reviewed-on: https://codeberg.org/calckey/calckey/pulls/9870
Co-authored-by: daikei <daikei@noreply.codeberg.org>
Co-committed-by: daikei <daikei@noreply.codeberg.org>