[Add todo] remove unnecessary specialization

dune/solvers/norms/twonorm.hh

@@ -32,6 +32,9 @@ class TwoNorm : public Norm<VectorType>
             return f.two_norm2();
         }
 
+        // TODO there is no reason to have this if f.two_norm2() can be
+        //      assumed (above) anyway. Then also the specialization for
+        //      FieldVector (below) can go.
         //! Compute the norm of the difference of two vectors
         virtual double diff(const VectorType& f1, const VectorType& f2) const
         {