[IMP] base: removes storing of res_name from ir.attachment

This commit removes the store property of `res_name` from `ir.attachment`
as it doesn't need to be stored and could contain outdated information
if the record name was changed after the computation.

Task: #1943295

closes odoo/odoo#34930

Signed-off-by: Martin Geubelle (mge) <mge@openerp.com>