[FIX] website, web_editor: fix empty parallax snippet not being removed

Previously, when removing all the content of a snippet, that snippet
would get automatically deleted. This was not the case for parallax
snippets because we previously intended the user to drop the parallax
snippet, empty it, and drop other content inside of it.

Recently, we added the parallax option on all snippets, rendering the
previous workflow obsolete. There is now no longer any reason to keep
empty parallax snippets. This commit fixes that.

task-2446008

closes odoo/odoo#64907

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