Delete non-migrated previews for regeneration

[skjnldsv]

Steps

1. have an old instance with 100k previews
2. upgrade to 20
3. see you have to manually migrate those old previews
4. see it takes you 100+ days to finish

Solution

Delete those non-migrated previews as it's faster to generate those on demand than move them
Especially between buckets.

Context

https://github.com/nextcloud/server/blob/215aef3cbdc1963be1bb6bca5218ee0a4b7f1665/core/Command/Preview/Repair.php