Remove 'blocksize' member from MultigridStep

It seems to have no functionality and is incompatible with MultiTypeBlockVector

dune/solvers/iterationsteps/multigridstep.hh

@@ -24,8 +24,6 @@ namespace Dune {
     class MultigridStep : public LinearIterationStep<MatrixType, VectorType, BitVectorType>
     {
 
-        static const int blocksize = VectorType::block_type::dimension;
-
     public:
         using LinearStepType = LinearIterationStep<MatrixType, VectorType, BitVectorType>;