removes rounded corners on images in posts

Meta: [Why did my image get rounded corners?](http://meta.discourse.org/t/why-did-my-image-get-rounded-corners/6310)

This removes the rounded corners applied on images in posts in the *default* theme.

Other forums might override it with:

```css
.topic-post article.boxed img {
  border-radius: 4px;
}
```