Fix on delete cascade entity order (#20949)

* Fix on delete cascade entity order

Since [#20708](https://github.com/netbox-community/netbox/pull/20708)
relation with a on delete RESTRICT are not deleted in the proper order.
Then the error `violate not-null constraint` occurs and breaks the
delete cascade feature.

* Revert unrelated and simplify changes

netbox/core/signals.py

@@ -3,7 +3,7 @@ from threading import local
 
 from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import ObjectDoesNotExist, ValidationError
-from django.db.models import CASCADE
+from django.db.models import CASCADE, RESTRICT
 from django.db.models.fields.reverse_related import ManyToManyRel, ManyToOneRel
 from django.db.models.signals import m2m_changed, post_migrate, post_save, pre_delete
 from django.dispatch import receiver, Signal
@@ -221,7 +221,7 @@ def handle_deleted_object(sender, instance, **kwargs):
             obj.snapshot()  # Ensure the change record includes the "before" state
             if type(relation) is ManyToManyRel:
                 getattr(obj, related_field_name).remove(instance)
-            elif type(relation) is ManyToOneRel and relation.null and relation.on_delete is not CASCADE:
+            elif type(relation) is ManyToOneRel and relation.null and relation.on_delete not in (CASCADE, RESTRICT):
                 setattr(obj, related_field_name, None)
                 obj.save()