[FIX] purchase_requisition: fiscal position tax mapping

`map_tax` must be called using the fiscal position
browse record, not using its id.

Besides, `fpos` is not used further in the code.

opw-670170

addons/purchase_requisition/purchase_requisition.py

@@ -143,7 +143,7 @@ class purchase_requisition(osv.osv):
         qty = product_uom._compute_qty(cr, uid, requisition_line.product_uom_id.id, requisition_line.product_qty, default_uom_po_id)
 
         taxes = product.supplier_taxes_id
-        fpos = supplier.property_account_position_id.id
+        fpos = supplier.property_account_position_id
         taxes_id = fpos.map_tax(taxes).ids if fpos else []
 
         po = po_obj.browse(cr, uid, [purchase_id], context=context)