Delete our webextensions for search engines when Bug 1885953 is fixed upstream

For #42891 (closed), we needed to rewrite our custom search engine customization.

As a result, web extensions should not be needed anymore, but upstream hasn't removed them yet.

They plan to do it with Bug 1885953.

We should do the same for a few search engines (ddg, ddg-onion, startpage, startpage-onion) once they do it. Therefore I've CC'ed myself to the Bug for now.