[BUG] mc admin group remove supports removing users

**Describe the bug**

https://min.io/docs/minio/linux/reference/minio-mc-admin/mc-admin-group.html#command-mc.admin.group

Current docs are out of date (or were always wrong) and state you can only remove a user from a group by removing the user.

`mc admin group remove` supports only removing users, not the whole group.
