Change how we hide SSO setting for windows

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=2001374

7c4705ca used to hide that setting with CSS.

At the moment, the setting is still hidden in our 148-based nightlies. However, I think the reason is that we're still using the old settings.

When actually switching to the new ones (with browser.settings-redesign.enabled), that setting reappears.