Remove unused "TanukiBot" code

What does this MR do and why?

• 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.
• The code here was using the old name and is no longer used
• Completes: Remove `TanukiBotService` and references (#431311 - closed)
• Partially completes: Rename completions templates to DuoChat (#431310) (there are other TanukiBot files that are still used but need to be renamed)

Screenshots or screen recordings

This is a refactor

MR acceptance checklist

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