Remove inexistent threads#list API

[ignacio-chiazzo]

The Threads API doesn't expose a method to get the list of threads. It isn't listed on the official documentation:

Trying that endpoint returns the following error:

"OpenAI HTTP Error (spotted in ruby-openai 6.3.1): {"error"=>{"message"=>"Your request to GET /v1/threads must be made with a session key (that is, it can only be made from the browser). You made it with the following key type: secret.", "type"=>"invalid_request_error", "param"=>nil, "code"=>"missing_scope"}}"

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[alexrudall]

Thanks, @ignacio-chiazzo - looks like it was undocumented and removed in November. I'm not call this a breaking change as it was not documented and was gone within a week of this method being added.

[alexrudall]

Released in v6.4