Remove the duo_chat_callout flag

What does this MR do and why?

The duo_chat_callout was introduced as a way to quickly mitigate potential QA failures when deploying the Duo Chat callout. The flag has always been enabled by default and was meant to be disabled only if things would go wrong with the callout.

After several months, the callout workers as expected and the flag can be removed. That's the crux of this MR.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #434708 (closed)