fs: remove custom Buffer pool for streams

The performance benefit of using a custom pool are negligable.
Furthermore, it causes problems with Workers and transferrable.
Rather than further adding complexity for compat with Workers,
just remove the pooling logic.

Refs: https://github.com/nodejs/node/issues/33880#issuecomment-644430693
Fixes: https://github.com/nodejs/node/issues/31733

PR-URL: https://github.com/nodejs/node/pull/33981


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>