always include "utf8.h" (instead of contrib/.../utf8.h)

code/AssetLib/STEPParser/STEPFileEncoding.cpp

@@ -45,11 +45,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 #include "STEPFileEncoding.h"
 #include <assimp/fast_atof.h>
-#ifdef ASSIMP_USE_HUNTER
-#  include <utf8.h>
-#else
-#  include <contrib/utf8cpp/source/utf8.h>
-#endif
+#include "utf8.h"
 
 #include <memory>