Remove Polymer

[Splaktar]

Angular 2.2 broke the Angular2-Polymer integration. Beyond that Angular2-Polymer doesn't support AOT, the latest Angular CLI, and more. Plus the Vaadin team has dropped their official support for the project (though thankfully there is still some community support working on a re-write).

I would prefer to wait for Polymer 2.0 (alpha) to reach GA before spending more time and energy on it. Material2 is in very good shape now, so we should be able to move pretty much all of the components over to it.