[FIX] contract: Delete cascade line wizard

For avoiding problems while trying to remove the contract/contract line

contract/wizards/contract_line_wizard.py

@@ -25,6 +25,7 @@ class ContractLineWizard(models.TransientModel):
         string="Contract Line",
         required=True,
         index=True,
+        ondelete="cascade",
     )
 
     @api.multi