Fix some linter errors and warnings

app/utils/group-by.js

@@ -2,7 +2,6 @@
 // Code from https://github.com/HeroicEric/ember-group-by (MIT licensed)
 //
 import { A } from '@ember/array';
-import { get } from '@ember/object';
 import { isPresent } from '@ember/utils';
 
 export default function groupBy (collection, property) {
@@ -11,11 +10,11 @@ export default function groupBy (collection, property) {
 
   if (items) {
     items.forEach(function(item) {
-      let value = get(item, property);
+      let value = item[property];
       let group = groups.findBy('value', value);
 
       if (isPresent(group)) {
-        get(group, 'items').push(item);
+        group.items.push(item);
       } else {
         group = { property: property, value: value, items: [item] };
         groups.push(group);