stream: fix todo

With the update to v8 6.1 this todo can now be addressed.

PR-URL: https://github.com/nodejs/node/pull/15667


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benedikt Meurer <benedikt.meurer@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>