test,worker: simplify common.isMainThread

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

PR-URL: https://github.com/nodejs/node/pull/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>