list > group > listitem should not be allowed

As of WAI-ARIA 1.2, groups are no longer allowed in lists. This is soon changing in ACT as well. We should make sure the following fails axe-core:

```html
<div role="list">
  <div role="group">
    <div role="listitem">list item</div>
  </div>
</div>
```

To fix this, remove `group` from `list` and `listitem` in standards/aria-roles.js, and fix any tests that fail.