Users shouldn't be able to pick a group as a subgroup of itself

Steps to reproduce:

1. Log in as Admin
2. Access Administration > Users & Rights > Groups
3. Edit a group
4. Click on the text box from "Subgroups to add" to access the picker
5. Observe the results in the picker
6. Choose the same group that is being edited

Expected results: The picker shows the groups available without the group that is being edited.

Actual results: The group that is being edited can also be chosen. The group then has itself as a subgroup.