Don't add CSS scoping attribute to elements that don't need it

The other side of the #678 coin — there's no sense in adding a `svelte-123xyz` attribute to an element that doesn't have a corresponding style:

```html
<div>
  <p>this should have the attribute, the parent should not</p>
</div>

<style>
  p {
    color: red;
  }
</style>
```