fix(): Zone already loaded, type implied (angular#1631)

I'm seeing complaints that angular#1599 didn't go far enough. It seems the Zone calls are fine being implicitly loaded; so let's drop the import completely.

Author: jamesdaniels

src/core/angularfire2.ts

@@ -7,8 +7,6 @@ import { first } from 'rxjs/operators';
 import firebase from '@firebase/app';
 import { FirebaseApp, FirebaseOptions, FirebaseAppConfig } from '@firebase/app-types';
 
-import {} from 'zone.js';
-
 export const FirebaseAppNameToken = new InjectionToken<string|undefined>('angularfire2.app.name');
 export const FirebaseOptionsToken = new InjectionToken<FirebaseOptions>('angularfire2.app.options');
 export const FirebaseAppConfigToken = new InjectionToken<FirebaseAppConfig|undefined>('angularfire2.app.config');