src: remove unused variable

PR-URL: https://github.com/nodejs/node/pull/18385


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>