Remove Deprecated `OpenAIAnswerGenerator` from the repo

[bilgeyucel]

OpenAIAnswerGenerator is deprecated and we should remove it from the codebase with the upcoming Haystack 1.23

[khareyash05]

Hey @bilgeyucel . I would like to contribute to this issue. Is it open?

[Amala02]

Hi! I would like to contribute to the issue as well. Let me know if its open!

[julian-risch]

@khareyash05 Yes, you can work on it if you want. You will need to open a PR against the v1.x branch. Not the main branch.

@Amala02 It's not a big issue so I wouldn't split the work between multiple people. Anyway, thank you for offering your help here. I am sure we have some other issues that might be interesting to you too. 🙂