Revert "Prevent_deref shall not return a reference to the iterator as it can be copied"

it seems to have dramatic consequences on the runtime when using Polyhedron in test_pmp_clip
for example.

This reverts commit 7c4123152b.

STL_Extension/include/CGAL/iterator.h

@@ -43,8 +43,6 @@ class Prevent_deref
   Prevent_deref<I>
   , I // base
   , I // value
-  , boost::use_default
-  , I
   >
 {
 public:
@@ -52,8 +50,6 @@ public:
   Prevent_deref<I>
   , I // base
   , I // value
-  , boost::use_default
-  , I
   > Base;
   typedef typename Base::reference reference;
   typedef typename std::pair<I, I> range;