Typesystem schema simplification

Follow-up of !241 (merged)

APIStar used typesystem to parse OpenAPI documents, and tried to parse many of its properties to provide a schema validation and documentation generation feature. We only use the API client part, which only uses a small part of the schema and does not need most of the schema definitions. We can clean those up, without immediately writing our own parser, as this will make it easier to see what is actually being used or not.

Testing whether removing a definition breaks something is as simple as running some POST/PUT/PATCH API request.