remove migrate_staff_users management command

This pre-2020 → post-2020 migration created a 'Staff users' Dojo_Group
seeded from is_staff users. Since OS no longer honors Dojo_Group
membership for authorization, the command now only writes inert rows
on OS-only deployments. Anyone still needing to run it has been on a
release with it for years; drop it as dead code.

Author: devGregA