Remove lock icon from admin mode setting

What does this MR do and why?

This code change updates removes the 🔒 icon from the label of the Admin Mode setting. By removing this supplemental icon, we can avoid confusing this setting with the pattern for cascading settings.

Related &13582

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to /admin/application_settings/general
2. Scroll to Sign-in restrictions and select Expand
3. Find the Admin Mode setting and note the removed 🔒 icon