Remove (index|integer)_sequence fallback to fix #73

All supported compiler/standard library combinations have it (so that this change does no harm).

Libc++, however, does not declare __cpp_lib_integer_sequence, causing the check to fail and the fallback to be used. This leads to problems because code is already freely mixing Dune::Std::*_sequence and std::*_sequence, expecting them to be the same, which with Libc++ they are not.