Removes service_desk_custom_email feature flag

What does this MR do and why?

This MR removes the 🎏 feature flag service_desk_custom_email. It's part of the Service Desk custom email iteration plan. The feature flag is enabled by default. See the feature flag rollout issue.

Contributes to #387003 (closed)

Screenshots or screen recordings

🚫 No visual changes.

How to set up and validate locally

If that's needed, please ping me and I'll provide validation steps 🙂

MR acceptance checklist

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