worker: remove undocumented .onclose property

Remove setting of a 'close' event handler on MessagePort through the use
of an `.onclose` property. We don't use this convention anywhere else in
our codebase for 'close' events, this feature is undocumented, and we
don't test it.

PR-URL: https://github.com/nodejs/node/pull/25904


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>