[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#60507

X-original-commit: 32d0b1086b5db6c58c1e2891d8d77fafb9482de9
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/website_slides/static/src/js/slides_course_unsubscribe.js

@@ -105,6 +105,7 @@ var SlideUnsubscribeDialog = Dialog.extend({
 
     _onClickSubscriptionSubmit: function () {
         if (this.isFollower === this.getSubscriptionState()) {
+            this.destroy();
             return;
         }
         this._rpc({