This makes the "medium" populate of `res.users` take few seconds rather
than 2min30 on my machine.
The issue was that `write` (not in batch) is calling
`_subscribe_users_automatically` not in batch, which is particularly
slow.
`discuss_channel_nosubscribe` is removed because it was uneffective. The
condition was only on create, but the group change in
`_inverse_notification_type` was systematically triggering the
auto-subscribe outside of create anyway, which is what we want.
Part-of: odoo/odoo#142734
Sébastien Theys
c7b53b8f68