[FIX] mass_mailing: fix finding duplicate mails

Before the commit, the _get_seen_list() function in the mass_mailing module was
not able to correctly identify all the duplicate email addresses in a given mass
mailing. This was because the function chose and used only one way to find an
email address for each record in the mailing list, even though there are many
ways to find an email address for a record.

For example, a crm.lead record might have an email address in its partner_id
field, but it might also have an email address in its email_normalized field.
This can vary from record to record.

To fix this issue, the _get_seen_list() function was updated to only look at the
email address to which emails have already been sent, rather than trying to
fetch it from the record itself. This ensures that all duplicate emails are
correctly identified and that no duplicate emails are sent in the mass mailing.

Task-3234378

closes odoo/odoo#118220

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>