[FIX] mass_mailing: unlink traces when unlinking mailings

There is no need to keep traces when unlinking mailings, being mail or sms.
Indeed even with marketing automation unlinking mailings means traces will
be lost and won't serve any statistics or reporting purpose anymore.

LINKS

Task 2076366 (send now)
Task 2067873 (template access)
PR #37298
PR odoo/enterprise#5750

addons/mass_mailing/models/mailing_trace.py

@@ -31,7 +31,7 @@ class MailingTrace(models.Model):
     model = fields.Char(string='Document model')
     res_id = fields.Integer(string='Document ID')
     # campaign / wave data
-    mass_mailing_id = fields.Many2one('mailing.mailing', string='Mailing', index=True)
+    mass_mailing_id = fields.Many2one('mailing.mailing', string='Mailing', index=True, ondelete='cascade')
     campaign_id = fields.Many2one(
         related='mass_mailing_id.campaign_id',
         string='Campaign',