A chained_struct 'field' in IDL is another IDL type whose fields are added directly to the BSON of the containing IDL type, instead of adding the contained type as an independent struct. It's basically the IDL version of inheritance.

A chained_type is a fundamental type which is treated the same way. This doesn't make a lot of sense, which is probably why it's completely unused.

Purge support for chained_type so we don't have to maintain it.