test,worker: simplify common.isMainThread

Now that `worker_threads` do not require a flag, the logic around
loading `isMainThread` can be removed.

PR-URL: #25426
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/common/index.js

@@ -46,13 +46,7 @@ const noop = () => {};
 
 const hasCrypto = Boolean(process.versions.openssl);
 
-const isMainThread = (() => {
- if (require('module').builtinModules.includes('worker_threads')) {
- return require('worker_threads').isMainThread;
- }
- // Worker module not enabled → only a single main thread exists.
- return true;
-})();
+const { isMainThread } = require('worker_threads');
 
 // Check for flags. Skip this for workers (both, the `cluster` module and
 // `worker_threads`) and child processes.