Chore: remove the feature flag default

What does this MR do and why?

This code change updates the AI model used by GitLab's AI-powered features. It switches the default model from Claude 3 to Claude 3.5, which is a newer and more advanced version. This change is expected to improve the quality and accuracy of the AI-generated responses. Additionally, the code removes the ability to specify a custom model, ensuring that all users are using the latest and best model available.

Ref:#469499

Changelog: removed EE: true

MR acceptance checklist

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Screenshots or screen recordings

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Before	After
Trace using Claude 3

How to set up and validate locally

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Related to #469499 (closed)