fix pyflake errors

review164006

product.py

@@ -1,6 +1,6 @@
 #This file is part of Tryton.  The COPYRIGHT file at the top level
 #of this repository contains the full copyright notices and license terms.
-from trytond.model import ModelView, ModelSQL, fields
+from trytond.model import ModelView, ModelSQL
 from trytond.transaction import Transaction
 from trytond.pool import Pool