Consider removing the orphaned ticket stats entry

[malemburg]

https://ep2021.europython.eu/admin/conference/conference/ep2021/stats/details?code=tickets_status.orphan_tickets

This uses an old query which no longer accurately describes the current system's behavior: tickets being associate to users via email address. This was the status many years ago, but we now have tickets reference the user object directly.

[hypha]

Some mysterious badge printing issue we encountered in 2019 might be related to the orphaned tickets: #1106
It'd be good to first get an idea of the underlying issue before removing the orphaned tickets completely.