remove support for Dune 2.2

this has slowly become a hassle to support (i.e., it cluttered the
source with many #if's and in particularly the code was not tested
with Dune 2.2 on a regular basis). Also, Dune 2.3 has been out since
more than two years, so IMO it is not asked too much to ask people who
want to use the latest and greatest version of ewoms to upgrade their
Dune.

tests/models/test_quadrature.cpp

@@ -270,13 +270,9 @@ void testQuadrature()
 {
     std::cout << "testing SCV quadrature...\n";
 
-#if DUNE_VERSION_NEWER(DUNE_COMMON, 2, 3)
     std::bitset<dim> isPeriodic(false);
     std::array<int, dim> cellRes;
-#else
-    Dune::FieldVector<bool, dim> isPeriodic(false);
-    Dune::FieldVector<int, dim> cellRes;
-#endif
+
     std::fill(cellRes.begin(), cellRes.end(), 10);
 
     GlobalPosition upperRight(1.0);
@@ -296,11 +292,7 @@ void testQuadrature()
 #endif
 
     // compute approximate integral
-#if DUNE_VERSION_NEWER(DUNE_COMMON, 2, 3)
     auto gridView = grid.leafGridView();
-#else
-    auto gridView = grid.leafView();
-#endif
     auto eIt = gridView.begin<0>();
     const auto eEndIt = gridView.end<0>();
     Scalar result = 0;

tests/test_blackoilfluidstate.cc

@@ -37,11 +37,7 @@
 
 // include dune's MPI helper header
 #include <dune/common/version.hh>
-#if DUNE_VERSION_NEWER(DUNE_COMMON, 2,3)
 #include <dune/common/parallel/mpihelper.hh>
-#else
-#include <dune/common/mpihelper.hh>
-#endif
 
 #include <opm/common/utility/platform_dependent/reenable_warnings.h>