fs: remove needless assignment of null

This line `pool = null;` isn't needed and has
been around since the first iteration of streams.
I can't find a good reason for it to exist, it's
not more readable, nor does it seem to trick the
compiler into any optimizations.

PR-URL: https://github.com/nodejs/node/pull/10260


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>