Remove std::is_literal_type from tests.

* It was deprecated in C++17 and removed in C++20

test/iso_week/year_weeknum.pass.cpp

@@ -65,7 +65,6 @@ static_assert( std::is_trivially_move_assignable<iso_week::year_weeknum>{}, "");
 
 static_assert(std::is_trivially_copyable<iso_week::year_weeknum>{}, "");
 static_assert(std::is_standard_layout<iso_week::year_weeknum>{}, "");
-static_assert(std::is_literal_type<iso_week::year_weeknum>{}, "");
 
 static_assert( std::is_nothrow_constructible<iso_week::year_weeknum,
                                                  iso_week::year,