Removing chat mutation flag

What does this MR do and why?

This change removes one feature flag from a list of AI-related features being sent to GitLab's AI Gateway. Specifically, it removes the "duo_workflow_chat_mutation_tools" feature flag while keeping all the other AI workflow features intact. This suggests that the chat mutation tools feature is either being discontinued, moved elsewhere, or no longer needs to be controlled by this particular feature flag system.

This feature flag has been released to everyone and the corresponding code in the Duo Workflow Service codebase has been removed.

Code removed in this MR: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!2773 (merged)

References

#538224 (closed)

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