Remove support for dark street and hybrid aerial tiles
Currently we only have access to the "regular" OSM Mapnik style tiles.
As it seems unlikely we will re-enable a dark street tile set, but rather eventually switch to vector tiles, and then implement dark view using a stylesheet we could remove support for the "streetDark" tile set to avoid having to port this code to libshumate.
The same goes for the "hybrid aerial" tiles that we used from Mapbox to implement "Show labels" in the aerial view. It seems more likely that a possible future return of this feature would use vector tiles overlayed ontop of a raster aerial base map.