Drop also foreign key constraint of old field column of ir.rule

The foreign key must be dropped otherwise changes in the ir.model.field table
can fail because of the foreign key constraint.
The migration should always remove all unused constraints.

issue5241
review17741002

trytond/ir/rule.py

@@ -117,6 +117,7 @@ class Rule(ModelSQL, ModelView):
 
         # Migration from 2.6: replace field, operator and operand by domain
         table.not_null_action('field', action='remove')
+        table.drop_fk('field')
         table.not_null_action('operator', action='remove')
         table.not_null_action('operand', action='remove')