[REF] mail: remove notified_partner_ids from _message_read_dict_postprocess

notified_partner_ids, (previously notified_partner_ids) is used to
populate a list of partner to prefetch + return a list of partner_ids.

1. the list of partner_ids is doesn't look to be used, and thus
can be removed. Was introduced in 1d4dbc34

2. Prefetched list of partners is used to generate customer_email_data.
Since this list is limited to partner email notifications , we can use
a custom search instead.

The proposed implementation is far from perfect but proposes the minimal
changes to remove notified_partner_ids, without (hopefully) negatively
impacting performances.

Task-ID 402597