test: remove experimental-wasm-threads flag

It is already true by default in V8.

Refs: v8/v8@be9ff65
PR-URL: #45074
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

test/parallel/test-worker-message-port-wasm-threads.js

@@ -1,4 +1,3 @@
-// Flags: --experimental-wasm-threads
 'use strict';
 const common = require('../common');
 const assert = require('assert');