remove composited layers

https://github.com/mapbox/mapbox-gl-js/issues/523#issuecomment-51731405

- [x] update spec https://github.com/mapbox/mapbox-gl-style-spec/pull/140
- [x] update example styles https://github.com/mapbox/mapbox-gl-styles/pull/14
- [x] remove in native https://github.com/mapbox/mapbox-gl-native/pull/420