Remove duo_session_plan_section feature flag

What does this MR do and why?

Removes the feature flag that gates the agent todos section in the session details info panel, which renders when a session emits todo_write tool calls.

References

Related to

• [FF] Roll out the agent todos section under `du... (#605922 - closed) • Allison Villa • 19.3
• Surface todo_write plan state in session detail... (#603503 - closed) • Emily Bauman, Allison Villa • 19.2

Previous MRs

• Add todo_write plan to session detail sidebar (!241946 - merged) • Allison Villa • 19.2
• Update agent todos section in session details (!244738 - merged) • Allison Villa • 19.2

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Prerequisite: Have GitLab Duo set up in your GDK
   • Have a Duo-enabled group and project in your GDK created by the Duo setup task above
2. Prerequisite: Have GitLab Runner working
3. Start a GitLab Duo agent flow session that produces a plan (the agent emits todo_write tool calls).
4. Open the session detail page and confirm the Agent todos section appears in the info sidebar, showing the checklist and the "X of Y completed" summary.
5. Open the same session in the AI side panel and confirm the Agent todos section also renders there.

MR acceptance checklist

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