Draft: Removes feature flag service_desk_custom_email_reply
What does this MR do and why?
Solves [Feature flag] Enable custom email reply address (#423880)
We kept the feature flag service_desk_custom_email_reply because M365 customers experienced errors with email processing. Instead of appending a comment to an existing Service Desk issue it created a new issue. This has been solved with Ingest Service Desk custom email address replies (!136443 - merged) (Custom email: Forwarding in Microsoft Exchange ... (#426269)), so we can remove the FF in this milestone.
The feature flag is used to gate the generation of the Service Desk email reply address. If enabled (default) it generates a reply address from the configured custom email address (if custom email is set).
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