writer callback: remove unused code

[syclik]

Remove
stan/src/stan/interface_callbacks/writer
stan/src/test/unit/interface_callbacks/writer

This should be trivial as these writers aren’t used for anything.

• leave csv.hpp in Stan: used by both RStan and CmdStan
• leave messages.hpp in Stan: used by both RStan and CmdStan

Remove:

• filtered_values.hpp. Move to RStan, not used in CmdStan
• noop.hpp
• sum_values.hpp. Move to RStan, not used in CmdStan
• values.hpp. Move to RStan, not used in CmdStan

[syclik]

That doesn't look right. They're used by the interfaces, so it's not a trivial remove. I'm pushing some of the writers to appropriate locations, like CmdStan, RStan, and PyStan.