Remove duo_agentic_chat_verification_check feature flag

What does this MR do and why?

Removes the duo_agentic_chat_verification_check feature flag, which has been enabled by default in production since 19.5.

References

#618773 (closed)

#602572 (closed)

Screenshots or screen recordings

Not applicable, no UI change.

How to set up and validate locally

Not applicable, flag removal only.

MR acceptance checklist

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