Delete an invoice after validation? (Odoo 8)

• Install the account_cancel module.
• Go to file:
  • sudo nano <odoo>/addons/account/account_invoice.py
•  Comment the function unlink like:

    @api.multi        
    def unlink(self):
    #for invoice in self:
        #if invoice.state not in ('draft', 'cancel'):
        #raise UserError(_('You cannot delete an invoice which is not draft or cancelled. You should refund it instead.'))        
        #elif invoice.move_name:
        #raise UserError(_('You cannot delete an invoice after it has been validated (and received a number). You can set it back to "Draft" state and modify its content, then re-confirm it.')) 
        return super(AccountInvoice, self).unlink()

• Restart Odoo server.
• Go to the accounting view and remove the canceled invoice(s).