stream: delete unused code

In implementation of `stream.Writable`, `writable._write()` is
always called with a callback that is `_writableState.onwrite()`.
And In `afterTransform()`, `ts.writechunk` and `ts.writecb` are
assigned to null.  So, `ts.writecb` is a true value if
`ts.writechunk` isn't null.

PR-URL: https://github.com/nodejs/node/pull/18278


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>