Remove external_note_author_service_desk feature flag

What does this MR do and why?

Remove external_note_author_service_desk feature flag

The feature that was gated by this flag, has been enabled in production for over 15 days now. It has not raised any alerts or regressions in prod. No impact on other features. Hence we are removing the FF and releasing the feature.

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Related to #408932 (closed)