Subject note deletion

Bengfort requested to merge subject-note-deletion into main

Subject notes have been deprecated since 0.53.0. We turned them read-only (!1646 (merged)) and provided a way for manual migration (!1764 (merged)).

Unfortunately we forgot to allow to delete the subject notes once they were migrated. So this is restores a way to delete subject notes.

I based this on !2001 (merged) to reduce complexity.

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