Strip Hexagon base, make invisible.

There's little point in maintaining the appearance of the base hexagon theme as it will only serve as the foundation. Next up is to make it invisible or not selectable. The only exposed themes will be the starter theme and Canvas when that's ported.

The styles, images and templates for hexagon base will be removed. It will only host the base foundation and a handful of plug-ins. The real customizations should happen in sub-themes.