buffer: removed unneeded FastBuffer constructor

PR-URL: https://github.com/nodejs/node/pull/19684


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jackson Tian <shyvo1987@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>