Fixed #23076, #25505 -- Fixed deletion of intermediate proxy models.

Thanks to James Murty for his work on an alternate patch.

tests/proxy_model_inheritance/models.py

@@ -6,7 +6,12 @@ class ConcreteModel(models.Model):
     pass
 
 
-class ConcreteModelSubclass(ConcreteModel):
+class ProxyModel(ConcreteModel):
+    class Meta:
+        proxy = True
+
+
+class ConcreteModelSubclass(ProxyModel):
     pass