[FIX] website_slides: statistics recompute when toggle website published

As a portal user, complete some steps within a course
Completing all of them makes the course completion to 100 %

Now, in the backend, unpublish one of these steps

Before this commit, the completion was not taking into account
that a step did not "exist" anymore i.e. has been unpublished,
leading to a completion > 100%

After this commit, we search only for steps that are published
The completion cannot exceed 100%

OPW 2033553

closes odoo/odoo#34800

Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>