fix: remove useless return

Author: fisker

doc/api/worker_threads.md

@@ -1710,7 +1710,6 @@ if (isMainThread) {
     worker.postMessage('hi');
     console.log(worker.performance.eventLoopUtilization());
   }, 100).unref();
-  return;
 } else {
   parentPort.on('message', () => console.log('msg')).unref();
   (function r(n) {