Remove `404.html`

Providing a `404.html` page doesn't make much sense for this project as users will need to configure their server in order for the page to be actually used.

In HTML5 Boilerplate, the idea is that the server configs (Apache or the alternatives) will basically ensure that the user's server will return the `404.html` page in case of an 404 error.

Also, with HTML5 Boilerplate, we've seen that users quite often forget to change/replace the default 404 page.