Remove product_suppliers domain constraint on product

As product suppliers can be defined at the product and template level but also
using a MatchMixin, it is no more flexible to have a domain using it on the
product. It does not hurt the data integrity because if the field is changed
the product_supplier field will also be validated (because it depends on it).
Also the on_change's ensure a default integrity for the user.

issue6799

purchase.py

@@ -946,10 +946,7 @@ class PurchaseLine(sequence_ordered(), ModelSQL, ModelView):
         ondelete='RESTRICT',
         domain=[
             If(Eval('purchase_state').in_(['draft', 'quotation']),
-                [('purchasable', '=', True),
-                    If(Bool(Eval('product_supplier')),
-                        ('product_suppliers', '=', Eval('product_supplier')),
-                        ())],
+                [('purchasable', '=', True)],
                 []),
             ],
         states={