[FIX] web_editor, *: correctly find `o_snippet_invisible`

*: website

This commit is a safety nest to ensure `o_snippet_invisible` will be
found, even if somehow they appear as one of the editable area elements.

It might be the case with some refactoring, as it was the case between
404fa816 and 83d99941c, in which case, the step to reproduce were:
  - Add s_popup snippet into the page
  - In the options, select 'Show On All pages' -> It moves to footer
  - Now save and enter Edit mode again
  - The invisible element (popup in this case) is in the DOM, the widget
    is working fine but it is not retrieved as an invisible element,
    thus not possible to edit it

+ lint/dead code

Part of https://github.com/odoo/odoo/pull/41294
task-2087003