Allow admin to remove devices of users which do not exist anymore

When being logged in as an admin of the seafile server, I can see multiple devices (last access dating years ago) which are associated with a user which does not exist anymore (the account has been deleted). If I try to delete the device, I simply get the error message “user invalid.”. It would be nice to be able to remove these devices.

(Tested using Seafile server 6.0.8)