test: remove unnecessary V8 flag

Refs: https://chromium-review.googlesource.com/c/v8/v8/+/2741582

PR-URL: #37671
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>

test/parallel/test-weakref.js

@@ -1,6 +1,6 @@
 'use strict';
 
-// Flags: --expose-gc --harmony-weak-refs
+// Flags: --expose-gc
 
 require('../common');
 const assert = require('assert');