build: remove entt library

include/entt/stl/cstddef.hpp

@@ -1,21 +0,0 @@
-#ifndef ENTT_STL_CSTDDEF_HPP
-#define ENTT_STL_CSTDDEF_HPP
-
-/*! @cond ENTT_INTERNAL */
-#if __has_include(<entt/ext/stl/cstddef.hpp>)
-#    include <entt/ext/stl/cstddef.hpp>
-#else
-#    include <cstddef>
-
-namespace entt::stl {
-
-using std::byte;
-using std::nullptr_t;
-using std::ptrdiff_t;
-using std::size_t;
-
-} // namespace entt::stl
-#endif
-/*! @endcond */
-
-#endif