Remove references to deprecated options

What does this MR do and why?

In Combine 3 sign-in settings (!124461 - merged), both:

1. ApplicationSetting.sign_in_text (Admin > Settings > General > Sign-in restriction > Sign-in text) and
2. ApplicationSetting.help_text (Admin > Settings > Preferences > Sign-in and Help page > Additional text to show on the sign-in page)

have been combined with Appearance.description (Admin > Settings > Appearance > Sign in/Sign up pages > Description).

Here we remove the documentation to these two deprecated options.

Final removal in %17.0: Combine sign-in appearances settings (#410885 - closed)

Screenshots or screen recordings

How to set up and validate locally

1. Check https://gdk.test:3443/help/api/settings.html

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