Remove cxx feature checks that are already in c++17 standard library
Summary
This MR removes some c++ feature checks from cmake, since the features are already standard:
DUNE_HAVE_CXX_BOOL_CONSTANT
DUNE_HAVE_CXX_EXPERIMENTAL_BOOL_CONSTANT
DUNE_HAVE_HEADER_EXPERIMENTAL_TYPE_TRAITS
DUNE_HAVE_CXX_APPLY
DUNE_HAVE_CXX_EXPERIMENTAL_APPLY
HAVE_IS_INDEXABLE_SUPPORT
The fallback implementations are removed and the inclusion into the Dune::Std
namespace is marked deprecated. This does typically not work for using
declarations. Thus, the functions are marked deprecated in Doxygen only.
Edited by Simon Praetorius