[FIX] mail: revert upload_attachment fix

Cookies should be used internally by the web UI. The server-side is not supposed to be aware of it at all.

Reverts:
https://github.com/odoo/odoo/pull/88745

Based on https://github.com/odoo/odoo/pull/93812


discussion. It has been decided to revert the fix to avoid further unattended behaviours.

closes odoo/odoo#100075

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>