Allow to delete invoices linked to sale and purchase
Because the lines links are doubled by a many2many ("sale.sale-account.invoice",
etc.). The code forbids to delete invoices. So the many2many link should be
removed and delete allowed.
Because the lines links are doubled by a many2many ("sale.sale-account.invoice",
etc.). The code forbids to delete invoices. So the many2many link should be
removed and delete allowed.