[build] move default_styles to platform/default

include/mbgl/util/default_styles.hpp

@@ -1,31 +0,0 @@
-#pragma once
-
-#include <vector>
-#include <string>
-
-namespace mbgl {
-namespace util {
-namespace default_styles {
-
-struct DefaultStyle {
-    const char* url;
-    const char* name;
-};
-
-extern const DefaultStyle streets;
-extern const DefaultStyle outdoors;
-extern const DefaultStyle light;
-extern const DefaultStyle dark;
-extern const DefaultStyle satellite;
-extern const DefaultStyle satelliteStreets;
-
-const DefaultStyle orderedStyles[] = {
-    streets, outdoors, light, dark, satellite, satelliteStreets,
-};
-const size_t numOrderedStyles = sizeof(orderedStyles) / sizeof(DefaultStyle);
-
-static const unsigned currentVersion = 9;
-
-} // end namespace default_styles
-} // end namespace util
-} // end namespace mbgl