[FIX] website_slides: correctly close subscription modal when saving

This commit fixes a small issue with the modal displayed when the user wants to
manage his subscription to a course.

When pressing the "Save" button, if no modification was made, the modal would
not close and nothing would happen (which seems strange on a user POV).
Now the modal correctly destroys itself and closes.

Task 2363028

closes odoo/odoo#60412

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