Hi @luca.guindani,
You can read about the media re-purposing feature here:
https://knowledge.kaltura.com/media-repurposing
The MRP custom metadata schema is auto added by this script /opt/kaltura/app/deployment/updates/scripts/addMediaRepurposingProfiles.php
Even if you don’t intend to use that, I recommend you do not remove it although doing so should not cause any damage.