Nuke a TODO

We use lumping for the nonlinearity, thus we only have diagonal entries

dune/tectonic/globalnonlinearity.hh

@@ -34,7 +34,6 @@ class GlobalNonlinearity {
   virtual shared_ptr<LocalNonlinearity<dim> const> restriction(int i) const = 0;
 
   void addHessian(VectorType const &v, MatrixType &hessian) const {
-    // TODO: is this correct?
     for (size_t i = 0; i < v.size(); ++i) {
       auto const res = restriction(i);
       res->addHessian(v[i], hessian[i][i]);