Remove a few lines of bogus code

They obviously do nothing (they add zero to the matrix diagonal).
Must be some left-overs from previous experimenting.

dune/solvers/solvers/umfpacksolver.hh

@@ -90,10 +90,6 @@ public:
       //   Modify Dirichlet rows
       ///////////////////////////////////////////
 
-      for (size_t j=0; j<modifiedStiffness.N(); j++)
-        for (int k=0; k<blocksize; k++)
-          modifiedStiffness[j][j][k][k] += 0.0;
-
       for (size_t j=0; j<modifiedStiffness.N(); j++)
       {
         auto cIt    = modifiedStiffness[j].begin();