Remove unneeded check in item-list template

If the `isHiddenInList` property is changed to true (e.g. via a
computed property), then that view is removed, and its computed
properties are unhooks — so the `isHiddenInList` property won’t update
and the view can’t be shown again. Items should be hidden via CSS
instead.

framework/core/ember/app/templates/components/ui/item-list.hbs

@@ -1,9 +1,7 @@
 {{#each item in listItems}}
-  {{#unless item.isHiddenInList}}
-    {{#if item.isListItem}}
-      {{view item}}
-    {{else}}
-      <li class="{{item.listItemClass}}">{{view item}}</li>
-    {{/if}}
-  {{/unless}}
+  {{#if item.isListItem}}
+    {{view item}}
+  {{else}}
+    <li class="{{item.listItemClass}}">{{view item}}</li>
+  {{/if}}
 {{/each}}