[bugfix] GCC was giving too many deprecation warnings for enable_if

I still don't understand why, but GCC was spilling deprecation warnings
for Dune::enable_if even if we were only using std::enable_if.

Thus I removed the deprecation of the enable_if struct and only
deprecate the type within. That should still be enough as this
deprecation warning gets triggered when enable_if is used to aid in
overload resolution and the boolean parameter of enable_if is true.

dune/common/typetraits.hh

@@ -136,11 +136,11 @@ namespace Dune
   };
 
   template<bool B, class T = void>
-  struct DUNE_DEPRECATED_MSG("Use std::enable_if instead!") enable_if
+  struct enable_if
   {};
 
   template<class T>
-  struct DUNE_DEPRECATED_MSG("Use std::enable_if instead!") enable_if<true,T>
+  struct enable_if<true,T>
   {
     typedef DUNE_DEPRECATED_MSG("Use std::enable_if instead!") T type;
   };