[cleanup] Don't store unused references to MPIHelper singleton

This removes some unused variable warnings.

dune/functions/common/test/concept.cc

@@ -105,7 +105,7 @@ struct FooBar
 int main ( int argc, char **argv )
 try
 {
-  Dune::MPIHelper& helper = Dune::MPIHelper::instance(argc, argv);
+  Dune::MPIHelper::instance(argc, argv);
 
   Dune::Functions::TestSuite test;
 

dune/functions/common/test/differentiablefunctiontest.cc

@@ -224,7 +224,7 @@ struct DerivativeRangeTerminationTest
 int main ( int argc, char **argv )
 try
 {
-  Dune::MPIHelper& helper = Dune::MPIHelper::instance(argc, argv);
+  Dune::MPIHelper::instance(argc, argv);
   bool passed = true;
 
   passed = passed and DifferentiableFunctionImplementableTest::check();

dune/functions/functionspacebases/test/gridviewfunctionspacebasistest.cc

@@ -377,7 +377,7 @@ void testOnHybridGrid()
 
 int main (int argc, char* argv[]) try
 {
-  Dune::MPIHelper& helper = Dune::MPIHelper::instance(argc, argv);
+  Dune::MPIHelper::instance(argc, argv);
 
   testOnStructuredGrid<1>();
   testOnStructuredGrid<2>();

dune/functions/functionspacebases/test/taylorhoodbasistest.cc

@@ -18,7 +18,7 @@ using namespace Dune::Functions;
 
 int main (int argc, char* argv[]) try
 {
-  Dune::MPIHelper& helper = Dune::MPIHelper::instance(argc, argv);
+  Dune::MPIHelper::instance(argc, argv);
   // Generate grid for testing
   const int dim = 2;
   typedef YaspGrid<dim> GridType;

dune/functions/gridfunctions/test/analyticgridviewfunctiontest.cc

@@ -30,7 +30,7 @@ double x_component_B(const Coord & x)
 
 int main (int argc, char* argv[]) try
 {
-  Dune::MPIHelper& helper = Dune::MPIHelper::instance(argc, argv);
+  Dune::MPIHelper::instance(argc, argv);
   // Generate grid for testing
   const int dim = 2;
   typedef YaspGrid<dim> GridType;

dune/functions/gridfunctions/test/discreteglobalbasisfunctiontest.cc

@@ -49,7 +49,7 @@ bool checkInterpolationConsistency(B&& basis, C&& x)
 
 int main (int argc, char* argv[]) try
 {
-  Dune::MPIHelper& helper = Dune::MPIHelper::instance(argc, argv);
+  Dune::MPIHelper::instance(argc, argv);
   bool passed = true;
 
   // Generate grid for testing

dune/functions/gridfunctions/test/gridfunctiontest.cc

@@ -28,7 +28,7 @@ using namespace Dune::Functions;
 
 int main (int argc, char* argv[]) try
 {
-  Dune::MPIHelper& helper = Dune::MPIHelper::instance(argc, argv);
+  Dune::MPIHelper::instance(argc, argv);
 
   // Generate grid for testing
   const int dim = 2;