[FIX] account_facturx: cleanup attachment management

Since 5cf9d55b, attachments are always records.
Before, they could also be named tuples, so helpers were needed.
Since they are now useless, we can remove them to clean up things.

closes odoo/odoo#38164

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>