stream: remove dead code

This condition could never be met because all calling functions
guarded against this.

PR-URL: https://github.com/nodejs/node/pull/15665


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>