IMediaLibrary: Remove media convertion callbacks

From the users' perspective, a media can be considered as removed from
the database as long as it's not part of any listing.
This simplifies the implementation for the clients, and matches the
initial intent of not providing explicit callbacks for external media.

Fix vlc#26854
8 jobs for !653 with simplify_media_callbacks in 7 minutes and 14 seconds (queued for 23 seconds)
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