Delete gh-pages branch

We should not have two copies (typically not both up to date) of the examples and API docs.  The [website readme](https://github.com/openlayers/openlayers.github.io/blob/build/README.md) has instructions for publishing examples and docs to what I think should be the canonical location.

My preference would be to delete the `gh-pages` branch entirely.  A second-best alternative (in my opinion) would be to add pages for things like the example index that would redirect.  But since we're not going to be able to swing a 301, I think 404 is the way to go.