src: remove unused `Worker::child_port_` member

This fixes a compiler warning introduced in 92259395.

PR-URL: https://github.com/nodejs/node/pull/31599


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>