accounting: only allow Assignor to restart accounting periods

This partially reverts 8a336dc5 (erp5_accounting: Allow
Assignor manage Accounting Periods, 2024-09-16) for the restart
transition, it is intentional that only Assignor can restart
an accounting period that have been closed.
The idea was to support a scenario where re-opening a period
that was closed can not be done directly by the Assignee but
needs validation from the assignor.