[FIX] website_slides: hide the website default header in fullscreen

Bug
===
You can enter in a fullscreen mode in the frontend, of the slide channel.
In this view, the default website header should not be visible.

To do that, we play with z-index, but since 0f2cab4a ,
the z-index of the website header has been changed without changing the z-index
of the fullscreen view.

Task-2345016

closes odoo/odoo#58284

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