stream: remove unreachable code

To avoid a function call `BufferList.prototype.concat()` is not called
when there is only a buffer in the list. That buffer is instead
accessed directly.

Backport-PR-URL: https://github.com/nodejs/node/pull/19483
PR-URL: https://github.com/nodejs/node/pull/18239


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