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.

applications/ebos/eclpeacemanwell.hh

@@ -609,11 +609,7 @@ public:
         assert(context.element().template count</*codim=*/dimWorld>() == 8);
 #endif
 
-#if DUNE_VERSION_NEWER(DUNE_COMMON, 2,3)
         const auto &refElem = Dune::ReferenceElements<Scalar, /*dim=*/3>::cube();
-#else
-        const auto &refElem = Dune::GenericReferenceElements<Scalar, /*dim=*/3>::cube();
-#endif
 
         // determine the current element's effective size
         const auto &elem = context.element();