Remove iterator-testing code redundant in C++20

[m-fila]

BEGINRELEASENOTES

• Removed redundant check for C++20
• Removed iterator test that is no longer required in C++

ENDRELEASENOTES

Some leftovers from #698
The testsstd::iterator_traits<It>::value_type were originally surrounded by #if (__cplusplus < 202002L) and could be removed

[m-fila]

I forgot to remove the redundant check from documentation. Should be good now