Delete old zimfarm-dispatcher-mongodb borgbase repository

[benoit74]

I think it is now more than time to delete the old zimfarm-dispatcher-mongodb borgbase repository. This is the old repository used when Zimfarm DB was still on MongoDB, more than one year ago. I don't think we have any chance to need this backup anymore, even for historical reasons. Or do we? Or should we keep one yearly backup just for "reference", should we want to dig into archives?

I think it could be a great opportunity to document what needs to be cleaned-up (we have also a key to delete in borgbase if I'm not mistaken, and corresponding credentials in bitwarden).

[rgaudin]

If it were just me I'd keep the very last archive. Just as a reference of its last state. It's less than 1G but I understand we'll probably never have any use of it and stuff piling up is a burden so I wont cry if we remove it altogether.