system: Replace dynamic System row subtitles with static text
Row subtitles in Settings are typically a description of the main label or setting. You can see this in Mouse & Touchpad, Privacy, Sharing, and Wi-Fi. For the System panel we did something a bit different and used the subtitle to show the status of each panel.
Given the standard use of subtitles elsewhere, this feels a bit surprising which, as a result, makes the panel harder to read. I think that a standard description would probably also be more useful than the current labels.
This also removes the thin CcSystemPage abstract class because it was only introduced to add the support for dynamic System row subtitle. Now this abstraction is no longer necessary.
Fixes #2776 (closed)