Remove obsolete FetchAdapter mixin

`ember-data` will use it `fetch()` automatically now, see emberjs/data#6082

Author: Turbo87

app/adapters/application.js

@@ -1,6 +1,5 @@
 import DS from 'ember-data';
-import AdapterFetch from 'ember-fetch/mixins/adapter-fetch';
 
-export default DS.RESTAdapter.extend(AdapterFetch, {
+export default DS.RESTAdapter.extend({
   namespace: 'api/v1',
 });