[FIX] website_sale_slides: remove buy button from already bought course

Currently, when trying to complete a quiz of an already bought course,
there is still a button to buy the course below the "check answers"
button. This commit removes the buy buttons for courses where the users
is not readonly (the user is readonly if he has not joined/bought the course).

closes odoo/odoo#37717

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