Remove duplicated $ref objects

[joellabes]

JSON Schema supports cross-file refs, but as far as I can tell, schemata installed through schemastore.org must be hosted on their GitHub repo to work. This depends on #3, so that we can decide how to solve it.

For now, the code just duplicates common objects like string_or_array_of_strings (1, 2).

[joellabes]

Closing this, as we'll keep the duplicates in favour of retaining this repo as the source of truth. If it ever becomes possible for schemastore to do refs with remote files then that's a bonus, but not essential