chore: removes deprecated assign polyfill

Author: Pixelik

addon/utils/mung-options-for-fetch.ts

@@ -1,4 +1,3 @@
-import { assign } from '@ember/polyfills';
 import { serializeQueryParams } from './serialize-query-params';
 import {
   Method,
@@ -13,7 +12,7 @@ import {
 export default function mungOptionsForFetch(
   options: AjaxOptions
 ): FetchOptions {
-  const hash = assign(
+  const hash = Object.assign({},
     {
       credentials: 'same-origin'
     },