Remove `TanukiBotService` and references

Context

Before the name "GitLab Duo" existed, we called Duo Chat "Tanuki Bot". As a result, there is a lot of code in gitlab-org/gitlab that references this old name.

Definition of done

I think that ee/app/services/llm/tanuki_bot_service.rb is unused.

Remove it and related tests (ee/spec/services/llm/tanuki_bot_service_spec.rb) and this line of code and see if anything breaks.

If it does, we can evaluate renaming it instead. But I would think that all Chat-related logic goes through https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/llm/chat_service.rb instead.

Testing

There are RSpec tests for AI code. Make sure that any unit tests for removed code are also removed. If there are references to the removed code in integration tests, see if those code paths are needed or can be eliminated.

To ensure that the real LLM requests still work, ask the reviewer of the MR to trigger CI jobs that run real AI requests: https://docs.gitlab.com/ee/development/ai_features/duo_chat.html#running-the-rspecs-tagged-with-real_ai_request