System administrators can add `Owner` to entity they created, but not remove all owners.

## Description

When you
1. are logged in as the system administrator
2. create an entity like an event notification
3. share it with **one** other user by making them an `Owner` (make sure to save the changes before the next step)
4. try to remove the collaborator

you can't actually remove the collaborator, because you get the following warning:


## Your Environment

* Graylog Version: Tested with `6.2`