refactor(core): cleanup unused `NgZone` factory

This factory was only used when using `coreBootstrap` / `coreLoadAndBoostrap`
which is not supported any more.

modules/@angular/core/src/application_module.ts

@@ -18,7 +18,6 @@ import {ComponentResolver} from './linker/component_resolver';
 import {DynamicComponentLoader, DynamicComponentLoader_} from './linker/dynamic_component_loader';
 import {ViewUtils} from './linker/view_utils';
 import {NgModule} from './metadata';
-import {NgZone} from './zone/ng_zone';
 
 let __unused: Type;  // avoid unused import when Type union types are erased
 
@@ -30,19 +29,6 @@ export function _keyValueDiffersFactory() {
   return defaultKeyValueDiffers;
 }
 
-export function createNgZone(parent: NgZone): NgZone {
-  // If an NgZone is already present in the parent injector,
-  // use that one. Creating the NgZone in the same injector as the
-  // application is dangerous as some services might get created before
-  // the NgZone has been created.
-  // We keep the NgZone factory in the application providers for
-  // backwards compatibility for now though.
-  if (parent) {
-    return parent;
-  }
-  return new NgZone({enableLongStackTrace: isDevMode()});
-}
-
 /**
  * A default set of providers which should be included in any Angular
  * application, regardless of the platform it runs onto.
@@ -59,11 +45,6 @@ export const APPLICATION_COMMON_PROVIDERS: Array<Type|{[k: string]: any}|any[]>
  */
 @NgModule({
   providers: [
-    {
-      provide: NgZone,
-      useFactory: createNgZone,
-      deps: <any>[[new SkipSelfMetadata(), new OptionalMetadata(), NgZone]]
-    },
     ApplicationRef_,
     {provide: ApplicationRef, useExisting: ApplicationRef_},
     Compiler,