src: remove unused variables in node_util.cc

Currently two warnings are generated regarding unused variables.
This commit removes the unused variables.

PR-URL: https://github.com/nodejs/node/pull/24717


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>