[IMP] mail: Allow user deletion if linked to mail.activity

Purpose
=======

Currently the on_delete='restrict' implicit constraint prevents
the user deletion in case it's linked to a mail.activity record.

Set the on_delete attribute to 'cascade' as the record is not
of any usage without the user anyway.

Taskid: 3222941
Part-of: odoo/odoo#114656

addons/mail/models/mail_activity.py

@@ -82,7 +82,7 @@ class MailActivity(models.Model):
     user_id = fields.Many2one(
         'res.users', 'Assigned to',
         default=lambda self: self.env.user,
-        index=True, required=True)
+        index=True, required=True, ondelete='cascade')
     request_partner_id = fields.Many2one('res.partner', string='Requesting Partner')
     state = fields.Selection([
         ('overdue', 'Overdue'),