clean up swh-scheduler model db

The current scheduler holds lots of information that are no longer necessary:

• beside load-nixguix recurrent tasks all other are no longer scheduled by this old model
• only oneshot tasks are interesting but no more than 2 months old (save code now, deposit, vault, ...)

Those can be archived (and later on be dropped if we don't read in those [1] This will make the current mechanisms that needs read/write in those tables faster.

• [1] we usually don't.

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Migrated from T3837 (view on Phabricator)