Fixed #28477 -- Stripped unused annotations on aggregation.

Also avoid an unnecessary pushdown when aggregating over a query that doesn't
have aggregate annotations.

django/db/models/expressions.py

@@ -405,6 +405,12 @@ class BaseExpression:
         )
         return clone
 
+    def get_refs(self):
+        refs = set()
+        for expr in self.get_source_expressions():
+            refs |= expr.get_refs()
+        return refs
+
     def copy(self):
         return copy.copy(self)
 
@@ -1167,6 +1173,9 @@ class Ref(Expression):
         # just a reference to the name of `source`.
         return self
 
+    def get_refs(self):
+        return {self.refs}
+
     def relabeled_clone(self, relabels):
         return self