Remove polyfills from all builds except for firebase.js (#1731)

Author: Feiyang1

packages/firebase/app/index.ts

@@ -15,6 +15,5 @@
  * limitations under the License.
  */
 
-import '@firebase/polyfill';
 import firebase from '@firebase/app';
 export default firebase;

packages/firebase/src/index.cdn.ts

@@ -26,6 +26,7 @@ For the CDN builds, these are available in the following manner
 https://www.gstatic.com/firebasejs/5.0.0/firebase-<PACKAGE>.js
 `);
 
+import '@firebase/polyfill';
 import firebase from '../app';
 
 import '../auth';