[1.9.x] Removed unused Aggregate.input_field.

Obsolete since 4a66a69239.

Backport of 1f84721fac from master

django/db/models/aggregates.py

@@ -27,10 +27,6 @@ class Aggregate(Func):
         c._patch_aggregate(query)  # backward-compatibility support
         return c
 
-    @property
-    def input_field(self):
-        return self.source_expressions[0]
-
     @property
     def default_alias(self):
         expressions = self.get_source_expressions()