Delete "Layer" space from the codebase

[Gankra]

<nical> Gankro: (re: Layer/Layout) back then the terminology in place was "layer" in webrender, but it didn't make much sense for the API where turned out to be layout coordinates. We weren't sure whether the layout coordinates at the API boundary were going to remain the same as the "layer" coordinates used internally so we just kept both names with a typedef.

<Gankro> nical: can we kill it with fire now?

<nical> yeah
let's just call it all layout