[css-break] Consider removing references to the CSS Regions spec

[Psychpsyo]

The CSS Fragmentation spec mentions CSS Regions in quite a few places. From what I can tell, CSS Regions are no longer supported in any browser and were never supported in all major browsers to begin with.
Can I use tells us that the last browser to 'remove' these was Edge, when it switched to Chromium in 2018. Before then, they never were in Firefox, were dropped by Chrome in 2013 and dropped by Safari in 2017.

So my question is:
Is there any reason for these references to stay around? They lead to some WPT test failures in every browser concerning the avoid-region value for breaking properties. (which are also listed as 'at risk' in the spec itself)

[fantasai]

It probably makes sense to drop them from Level 3, but one reason to keep them around in whatever is our latest spec is because it makes it clear how this is all going to work once we do get some kind of regions-like feature.

We do want to get there eventually. :)