hide_label() has no effect for fields in media library

When registering fields for attachments, `hide_label()` has no effect. The resulting CSS for hiding the label is 
```
div.acf-field.acf-field-{field-key} > div.acf-label > label { display: none; }
```
but the media library meta fields use table markup. `div.acf-label` could be changed to `.acf-label` to target all elements.