Do not reset non-existent fields on requisition in draft state

Closes #13532
(grafted from 35037337ef21b2510ce561803b07b7413bb72e95)

--HG--
branch : 6.0

modules/purchase_requisition/purchase.py

@@ -376,8 +376,6 @@ class PurchaseRequisition(Workflow, ModelSQL, ModelView):
     @reset_employee('approved_by', 'rejected_by')
     def draft(cls, requisitions):
         cls.write(requisitions, {
-                'tax_amount_cache': None,
-                'untaxed_amount_cache': None,
                 'total_amount_cache': None,
                 })