[FIX] website: remove divs created by default shadow computation

The shadow option needs to create a div on the fly to be able to
retrieve the default shadow value. This commit makes sure these divs
are also removed when no longer needed.

task-2752326

closes odoo/odoo#82223

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>