Remove duo_developer_model_config feature flag

What does this MR do and why?

Removes the duo_developer_model_config feature flag now that it has been enabled by default in !252653 (merged) and is fully rolled out.

The developer/v1 foundational flow now unconditionally uses the :duo_developer feature setting instead of falling back to :duo_agent_platform.

References

• Enabled by default: !252653 (merged)
• Introduced by: !251961 (merged)
• Rollout issue: #623124 (closed)

Screenshots or screen recordings

N/A — no UI changes.

How to set up and validate locally

No setup required. The feature flag is removed; the developer/v1 flow unconditionally routes to the duo_developer feature setting.

MR acceptance checklist

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