Prevent issues during destruction when parentView is observed.

Setting `parentView` to `null` is technically better, but unfortunately
we have not had this reset in place for a while and folks have dependent
keys that include `parentView`. Setting without `Ember.set` triggers the
mandatory setter assertion.

This removes the clearing of `parentView` (for now) so that we can
properly message it being unobservable in the future.

packages/ember-glimmer/tests/integration/components/curly-components-test.js

@@ -2565,4 +2565,19 @@ moduleFor('Components test: curly components', class extends RenderingTest {
 
     this.assertText('things');
   }
+
+  ['@test using parentView in the template does not error during destruction']() {
+    this.registerComponent('foo-bar', {
+      ComponentClass: Component.extend({
+        title: computed('parentView.title', function() {
+          return this.get('parentView.title');
+        })
+      }),
+      template: `{{title}}`
+    });
+
+    this.render(`{{foo-bar}}`, { title: 'things' });
+
+    this.assertText('things');
+  }
 });

packages/ember-htmlbars/lib/hooks/cleanup-render-node.js

@@ -18,7 +18,6 @@ export default function cleanupRenderNode(renderNode) {
       if (view.parentView && view.parentView === env.view) {
         view.parentView.removeChild(view);
       }
-      view.parentView = null;
 
       view._transitionTo('preRender');
     });