Remove Google Maps example

This PR removes the Google Maps example. I think this example makes people think they can use Google Maps with OpenLayers 3, which is not true.

I created a gist for that example instead, so we can point people to something (with all the necessary warnings):

* gist: https://gist.github.com/elemoine/e82c7dd4b1d0ef45a9a4
* bl.ocks: http://bl.ocks.org/elemoine/e82c7dd4b1d0ef45a9a4

I still want to add more information to the gist's README.md.