Remove default value for unknown request_quotations field

Closes #13212
(grafted from 51daf548936e2c4fe63bd3449ccdd204165cb85d)

--HG--
branch : 7.0

modules/purchase_request_quotation/purchase.py

@@ -226,7 +226,6 @@ class Quotation(Workflow, ModelSQL, ModelView):
             default = default.copy()
         default.setdefault('number', None)
         default.setdefault('revision', cls.default_revision())
-        default.setdefault('request_quotations', None)
         return super(Quotation, cls).copy(groups, default=default)
 
     @property