[FIX] website_slides: prevent copy of a attendee in course

Before this commit:

Duplicating the contact that is linked to an attendee of a course also
creates a copy of an attendee as well.

After this commit:

It does not create duplicate attendee in course.

Task-3253983

closes odoo/odoo#125324

X-original-commit: e6c4937437bdf5e55c48fc80faf65b5094c96879
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>