Purpose
=======
Improve the performance of mass mailing by removing the <mail.mail>, in
batch, all at once. The unlink operation is very expensive for the ORM,
in terms of SQL queries.
A new field `to_delete` is required to know which <mail.mail> we need
to delete. The reason is that the `failure_type` is stored on the
<mail.notification>, and it's possible to have <mail.mail> without
<mail.notification>.
Task-2587345
Part-of: odoo/odoo#73271
std-odoo
036a739b62