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: #25904
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>

lib/internal/worker/io.js

@@ -102,12 +102,6 @@ Object.defineProperty(MessagePort.prototype, onInitSymbol, {
 
 // This is called after the underlying `uv_async_t` has been closed.
 function onclose() {
-  if (typeof this.onclose === 'function') {
-    // Not part of the Web standard yet, but there aren't many reasonable
-    // alternatives in a non-EventEmitter usage setting.
-    // Refs: https://github.com/whatwg/html/issues/1766
-    this.onclose();
-  }
   this.emit('close');
 }