[css-sizing-4] Remove Not Ready warning. There's still stuff to fix, of course, but it's not a total mess anymore.

Author: fantasai

css-sizing-4/Overview.bs

@@ -3,7 +3,6 @@ Title: CSS Box Sizing Module Level 4
 Shortname: css-sizing
 Level: 4
 Status: ED
-Warning: not ready
 Work Status: Exploring
 Group: csswg
 ED: https://drafts.csswg.org/css-sizing