libregraph: DELETEing a school should delete all relations

[butonic]

When a school is deprovisioned, all relations to users and classes should be deleted as well.

Note: Users must not be deleted when they are no longer part of a school. They have to be deleted with explicit requests against the /users/{id | onpremisessamaccountname} or /education/users/{id | onpremisessamaccountname}