Assistants API changed upstream

I haven't enumerated what exactly OpenAI changed in the Assistants API behavior, but it's not working correctly and the quick fixes did not work for me. The functionality in `src/assistants.jl` needs to be fixed, along with the tests if necessary.

For now, I'm disabling the functions so we can release.

Here's a migration guide, also have not yet perused this: https://platform.openai.com/docs/assistants/migration