This reverts commit 62622011.

in 62622011, we wanted address the needs to use the `string_view`
converter in C++98, but that requirement was based on wrong
preconditions. `std::string_view` was introduced in C++17, and
popular standard libraries like libstdc++ and libc++ both provide
`std::string_view` when the source is built with C++17.

furthermore 62622011 is buggy. because it uses `<version>` to tell
the feature set provided by the standard library. but `<version>`
is a part of C++20. so this defeats the purpose of the change of
62622011.

Fixes #1223