Remove ill-formed MSVC-specific code that doesn't compile with /permissive-

include/boost/range/concepts.hpp

@@ -252,12 +252,7 @@ namespace boost {
                  n = i - j;
              }
          private:
- // MSVC 14.1 - avoid C4596: 'difference_type': illegal qualified name in member declaration
- #if defined(_MSC_VER) && _MSC_VER >= 1912 
-             BOOST_DEDUCED_TYPENAME difference_type n;
- #else
-             BOOST_DEDUCED_TYPENAME RandomAccessIteratorConcept::difference_type n;
- #endif
+             BOOST_DEDUCED_TYPENAME RandomAccessIteratorConcept<Iterator>::difference_type n;
              Iterator i;
              Iterator j;
  #endif