Clean up Straight_skeleton as Unique_hash_map is not used at all

Author: afabri

Straight_skeleton_2/include/CGAL/Straight_skeleton_2/Straight_skeleton_builder_2_impl.h

@@ -15,7 +15,6 @@
 
 #include <CGAL/number_type_config.h>
 #include <CGAL/Real_timer.h>
-#include <CGAL/Unique_hash_map.h>
 
 #include <boost/utility.hpp>
 #include <boost/version.hpp>