lib: remove useless statement

There's no need to set 'false' to 'isAsync' because we've declared it
before.

PR-URL: https://github.com/nodejs/node/pull/39983


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>