stream: remove unused _transformState

_transformState is no longer used since Transform was
simplified.

Refs: https://github.com/nodejs/node/pull/32763

PR-URL: https://github.com/nodejs/node/pull/33105


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