[FIX] website_slides: leaving a course unsubs from the related channel

Steps to reproduce:
- install eLearning
- go to the website > courses > enroll in any course > leave the course

Previous behavior:
you are not unsubscribed from the slide channel and still receive mail
notifications

Current behavior:
you are unsubscribed from the channel when you leave the course

opw-2244716

closes odoo/odoo#50404

X-original-commit: cb262738
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>
Signed-off-by: mightyjol <jhk-odoo@users.noreply.github.com>