Cannot delete external judgement type

[eldering]

Description of the problem

Cannot remove external judgement types in the admin configuration UI.

Your environment

Current main branch using WF Astana database dump.

Steps to reproduce

1. Go to http://localhost/domjudge/jury/config?diffs=%5B%5D#Externalsystems
2. Add an external judgement type
3. See that it exists at http://localhost/domjudge/api/contests/wf48_finals/judgement-types
4. Try to delete it again.

Expected behaviour

The judgement type is gone.

Actual behaviour

It's still there, both in the config UI and API.

[eldering]

Same issue as #2472: only the last one cannot be deleted.