Remove async-gc harness

This is now unused, and there is no way to implement it without using the
cleanupSome proposal (see previous commit) which has been withdrawn.

Closes: #3920

harness/async-gc.js

@@ -1,57 +0,0 @@
-// Copyright (C) 2019  Ecma International.  All rights reserved.
-// This code is governed by the BSD license found in the LICENSE file.
-/*---
-description: >
-    Collection of functions used to capture references cleanup from garbage collectors
-features: [FinalizationRegistry.prototype.cleanupSome, FinalizationRegistry, Symbol, async-functions]
-flags: [non-deterministic]
-defines: [asyncGC, asyncGCDeref, resolveAsyncGC]
----*/
-
-function asyncGC(...targets) {
-  var finalizationRegistry = new FinalizationRegistry(() => {});
-  var length = targets.length;
-
-  for (let target of targets) {
-    finalizationRegistry.register(target, 'target');
-    target = null;
-  }
-
-  targets = null;
-
-  return Promise.resolve('tick').then(() => asyncGCDeref()).then(() => {
-    var names = [];
-
-    // consume iterator to capture names
-    finalizationRegistry.cleanupSome(name => { names.push(name); });
-
-    if (!names || names.length != length) {
-      throw asyncGC.notCollected;
-    }
-  });
-}
-
-asyncGC.notCollected = Symbol('Object was not collected');
-
-async function asyncGCDeref() {
-  var trigger;
-
-  // TODO: Remove this when $262.clearKeptObject becomes documented and required
-  if ($262.clearKeptObjects) {
-    trigger = $262.clearKeptObjects();
-  }
-
-  await $262.gc();
-
-  return Promise.resolve(trigger);
-}
-
-function resolveAsyncGC(err) {
-  if (err === asyncGC.notCollected) {
-    // Do not fail as GC can't provide necessary resources.
-    $DONE();
-    return;
-  }
-
-  $DONE(err);
-}