Removes prefixer from border-radius shorthand mixin

Quick preface: The [border radius shorthand mixins](http://bourbon.io/docs/#border-radius) are _not_ just a prefix-ing mixin for CSS3 `border-radius`. That was [already deprecated](https://github.com/thoughtbot/bourbon/commit/88dd7ed2dcee3e9eb7c7b258a2e1d84ea9d69d96). However, these shorthand mixins were still calling on the prefixer mixin, even though only spec was being output.

No need for this! This PR removes calling the prefixer mixin.

I also think we should tweak the docs to be more clear:

- [x] Change the title to “Border radius shorthand”
- [x] Remove deprecation note (it’s old news, by now)
- [x] Show the CSS output

Update: [PR for changing docs](https://github.com/thoughtbot/bourbon/pull/569)

In case it’s not clear, this current `border-radius` mixin allows you to target both corners of the side of a box in one line:

```scss
@include border-right-radius(20px);
```

…which outputs:

```css
border-top-right-radius: 20px;
border-bottom-right-radius: 20px;
```