[FIX] account: fix traceback when resetting invoice to draft (#17262)

Due to a bad refactoring of this method during the remove of the
'report' module, a traceback occurs when resetting an invoice to draft.
This is due to the retrieve_attachment method that is used as an api.model
instead of api.multi.

addons/account/models/account_invoice.py

@@ -571,7 +571,7 @@ class AccountInvoice(models.Model):
             for invoice in self:
                 with invoice.env.do_in_draft():
                     invoice.number, invoice.state = invoice.move_name, 'open'
-                attachment = self.env['ir.actions.report'].retrieve_attachment(invoice.id)
+                    attachment = self.env.ref('account.account_invoices').retrieve_attachment(invoice)
                 if attachment:
                     attachment.unlink()
         return True

odoo/addons/base/ir/ir_actions_report.py

@@ -179,7 +179,7 @@ class IrActionsReport(models.Model):
     #--------------------------------------------------------------------------
     # Main report methods
     #--------------------------------------------------------------------------
-    @api.model
+    @api.multi
     def retrieve_attachment(self, record_id, attachment_name=None):
         '''Retrieve an attachment for a specific record.