src: remove unused variable

This fixes a compiler warning.

PR-URL: https://github.com/nodejs/node/pull/17150


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>