Remove WeakRef/prototype/deref/gc-cleanup-not-prevented-with-wr-deref.js

On all engines that still support cleanupSome, this test was hitting the
asyncGC.notCollected check in resolveAsyncGC(). That means that it was
just skipped, and the deref() part was never actually executed.

Given that it is not testing anything in practice, remove it.

test/built-ins/WeakRef/prototype/deref/gc-cleanup-not-prevented-with-wr-deref.js

@@ -1,37 +0,0 @@
-// Copyright (C) 2019 Leo Balter. All rights reserved.
-// This code is governed by the BSD license found in the LICENSE file.
-
-/*---
-esid: sec-weak-ref.prototype.deref
-description: WeakRef deref should not prevent a GC event
-info: |
-  WeakRef.prototype.deref ( )
-
-  ...
-  4. Let target be the value of weakRef.[[Target]].
-  5. If target is not empty,
-    a. Perform ! KeepDuringJob(target).
-    b. Return target.
-  6. Return undefined.
-features: [FinalizationRegistry.prototype.cleanupSome, WeakRef, host-gc-required]
-includes: [async-gc.js]
-flags: [async, non-deterministic]
----*/
-
-var deref = false;
-var wr;
-
-function emptyCells() {
-  var target = {};
-  wr = new WeakRef(target);
-
-  var prom = asyncGC(target);
-  target = null;
-
-  return prom;
-}
-
-emptyCells().then(function() {
-  deref = wr.deref();
-  assert.sameValue(deref, undefined);
-}).then($DONE, resolveAsyncGC);