Removed special cased zoom level for HiDPI

No need to load higher-resolution vector tiles for HiDPI displays.

Fixes #907.

src/mbgl/map/source.cpp

@@ -206,7 +206,6 @@ TileData::State Source::addTile(Map& map, uv::worker& worker,
 
 double Source::getZoom(const TransformState& state) const {
     double offset = std::log(util::tileSize / info.tile_size) / std::log(2);
-    offset += (state.getPixelRatio() > 1.0 ? 1 :0);
     return state.getZoom() + offset;
 }