If you absolutely positive there are NO needed mailboxes on that database, you can delete via adsiedit, though its unsupported and you would need to be careful.
https://www.petenetlive.com/kb/article/0001414
https://lonesomehacks.com/2017/05/14/how-to-force-removal-of-exchange-server-mailbox-database/
Make sure you have accounted for ALL the arbitration mailboxes and they moved to another DB:
https://blog.rmilne.ca/2018/03/19/arbitration-mailboxes-lay-of-the-land/