doc: remove unused imports from worker_threads.md

PR-URL: #54147
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Deokjin Kim <deokjin81.kim@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/worker_threads.md

@@ -290,10 +290,8 @@ the last one will try to communicate with the main thread.
 
 ```mjs
 import { fileURLToPath } from 'node:url';
-import { once } from 'node:events';
 import process from 'node:process';
 import {
-  isMainThread,
   postMessageToThread,
   threadId,
   workerData,
@@ -328,9 +326,7 @@ channel.onmessage = channel.close;
 ```
 
 ```cjs
-const { once } = require('node:events');
 const {
-  isMainThread,
   postMessageToThread,
   threadId,
   workerData,