[BUGFIX beta] Fix double container destroy

This code on the Engine Instance is not needed as it includes the
`ContainerProxyMixin`, which already takes care of destroying the 
container.

This duplicated destroy step was causing ember-cp-validations to
attempt to destroy validators twice. This destroy process tries to 
nullify some property with `o.set(‘model’, null)` on an object that is
already destroyed.

packages/ember-application/lib/system/engine-instance.js

@@ -129,14 +129,6 @@ const EngineInstance = EmberObject.extend(RegistryProxyMixin, ContainerProxyMixi
     this._super(...arguments);
   },
 
-  /**
-    @private
-  */
-  willDestroy() {
-    this._super(...arguments);
-    run(this.__container__, 'destroy');
-  },
-
   /**
     Build a new `Ember.EngineInstance` that's a child of this instance.