Remove not needed function call.

Author: WaVEV

django_mongodb/compiler.py

@@ -189,7 +189,7 @@ def _get_group_expressions(self, order_by):
         having_group_by = self.having.get_group_by_cols() if self.having else ()
         for expr in having_group_by:
             expressions.add(expr)
-        return self.collapse_group_by(expressions, having_group_by)
+        return expressions
 
     def _get_group_id_expressions(self, order_by):
         """Generate group ID expressions for the aggregation pipeline."""