Update _clearing.scss

When creating a clearing gallery using block-grid for presentation, some styles interfere with the presentation of clearing.
Example:
```html
<ul class="large-block-grid-4 clearing-thumbs" data-clearing>
<li><a href="#" class="th"><img src="myimage.jpg"></a></li>
<li><a href="#" class="th"><img src="myimage.jpg"></a></li>
<li><a href="#" class="th"><img src="myimage.jpg"></a></li>
<li><a href="#" class="th"><img src="myimage.jpg"></a></li>
<li><a href="#" class="th"><img src="myimage.jpg"></a></li> <!-- This thumbnail will appear on second line -->
<li><a href="#" class="th"><img src="myimage.jpg"></a></li>
<li><a href="#" class="th"><img src="myimage.jpg"></a></li>
</ul>
```
The style for the block-grid break the presentation of the clearing thumbnails with this selector .large-block-grid-4 > li:nth-of-type(4n+1) because the fith thumbnail will be cleared and floated to the left.