Removed unused Aggregate.input_field.

Obsolete since 4a66a69239.

django/db/models/aggregates.py

@@ -26,10 +26,6 @@ class Aggregate(Func):
                     raise FieldError("Cannot compute %s('%s'): '%s' is an aggregate" % (c.name, name, name))
         return c
 
-    @property
-    def input_field(self):
-        return self.source_expressions[0]
-
     @property
     def default_alias(self):
         expressions = self.get_source_expressions()