[selectors] Deprecate single-colon syntax per https://lists.w3.org/Archives/Public/www-style/2015Sep/0165

Author: fantasai

selectors/Overview.bs

@@ -818,14 +818,8 @@ Syntax</h4>
 	for the Level 1 &amp; 2 pseudo-elements
 	(''::before'', ''::after'', ''::first-line'', and ''::first-letter'').
 	This compatibility notation is not allowed any other <a>pseudo-elements</a>.
-
-	Issue:
-	<a href="https://hg.csswg.org/drafts/diff/8e32bf889326/selectors/Overview.bs">Tab proposes</a>
-	deprecating the old syntax and RECOMMENDing that authors not use it.
-	<!--
 	However, as this syntax is deprecated,
 	authors should use the Level 3+ double-colon syntax for these <a>pseudo-elements</a>.
-	-->
 
 	<a>Pseudo-elements</a> are <a>featureless</a>,
 	and so can't be matched by any other selector.