Remove obsolete check

dune/solvers/iterationsteps/multigridstep.cc

@@ -148,9 +148,6 @@ void MultigridStep<MatrixType, VectorType, BitVectorType>::preprocess()
 
     if (basesolver_)
     {
-        if (this->basesolver_ == NULL)
-            DUNE_THROW(SolverError, "You have not provided a base solver!");
-
         // If the base solver can ignore dofs give it the ignoreNodes field
         if (dynamic_cast<CanIgnore<BitVectorType>*>(this->basesolver_))
             dynamic_cast<CanIgnore<BitVectorType>*>(this->basesolver_)->ignoreNodes_ = ignoreNodesHierarchy_[0];