child_process: remove unused internal event

Tests pass without this event being emitted. Remove it.

PR-URL: https://github.com/nodejs/node/pull/53793
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

lib/internal/child_process.js

@@ -557,7 +557,6 @@ class Control extends EventEmitter {
   unrefCounted() {
     if (--this.#refs === 0 && !this.#refExplicitlySet) {
       this.#channel.unref();
-      this.emit('unref');
     }
   }