Removes prefixes 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:

- [ ] Change the title to “Border radius shorthand”
- [ ] Remove deprecation note (it’s old news, by now)
- [ ] Show the CSS output