per
https://meta.discourse.org/t/imap-support-for-group-inboxes/160588/39?u=martin
we have been planning to remove IMAP support for a while,
because of its low usage and adoption, high complexity, and maintenance
burden.
This commit removes all IMAP-related code, including models,
jobs, services, and frontend components.
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Co-authored-by: Régis Hanol <regis@hanol.fr>