Remove CSS helper classes

The following CSS helper classes get removed from `main.css`:

* Image replacement: `.ir`
* Hidden elements: `.hidden`
* Visually hidden elements: `.visuallyhidden`
* Visually hidden elements that are focussable via keybord: `.visuallyhidden.focusable`
* Invisible elements: `.invisible`
* Clearfix: `.clearfix`

These classes were removed because they don't provide enough use
cases within modern web development. Cases are mostly handled
indiviually by developers.

All code is provided in branch
[`v4`](https://github.com/h5bp/html5-boilerplate/tree/v4).

Reference #1472.
Closes #1475.