[FIX] purchase: reset reminder status on cancel

Steps to reproduce:

* Create PO
* Confirm Receipt Date
* Cancel PO
* Draft and Confirm again

Current behavior:

* Button for Confirm Receipt Date is not visible

Expected behavior:

* Button for Confirm Receipt Date should be visible

This is happening as we are not resetting the value of `mail_reminder_confirmed` on cancelling PO.

With this commit, we reset value of `mail_reminder_confirmed` so use can Confirm Receipt Date again.

closes odoo/odoo#59674

Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>