lib,src: remove usage of _externalStream

Since 4697e1b0, it is no longer
necessary to use `v8::External`s to pass `StreamBase` instances
to native functions.

PR-URL: https://github.com/nodejs/node/pull/26510
Refs: https://github.com/nodejs/node/pull/25142


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>