Prefetch specific model instances

This reduces the number of queries performed during delete, slightly.

wagtail/core/models/__init__.py

@@ -559,8 +559,8 @@ class Page(AbstractPage, index.Indexed, ClusterableModel, metaclass=PageBase):
                     user=user,
                     deleted=True,
                 )
-            for child in self.get_descendants():
+            for child in self.get_descendants().specific():
-                log_deletion(child.specific, user)
+                log_deletion(child, user)
             log_deletion(self.specific, user)
 
             # this is a Page instance, so carry on as we were