adapt/adt: Don't add/remove const on return type of attributes

Boost.TypeOf always deduces the type as un-cv-qualified value type, thus const-ized
value type is redundant and inhibiting compiler optimization.

test/sequence/adapt_adt.cpp

@@ -300,7 +300,7 @@ main()
         BOOST_MPL_ASSERT((
             boost::is_same<
                 boost::fusion::result_of::back<ns::point const>::type::type,
-                const int
+                int
             >));
     }