Bug
===
Since 6185f14807 we check the <mail.mail>
existence before marking the <mailing.trace> as opened, but since
57ae1b9b8b61f5f4719a8a81e9d0d21fab58cfda we remove the <mail.mail>
automatically when we send them.
The result is that this endpoint always raise a 500 error.
To be: the <mail.mail> existence shouldn't be checked in this endpoint
(the token is valid for the raw integer id).
Task-3234519
closes odoo/odoo#115842
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>