[css-values-4] Remove accidentally retained line.

Author: tabatkins

css-values-4/Overview.bs

@@ -2319,7 +2319,6 @@ Ian's proposal:
 	to be more generally about "substitute a reference"
 	and just use that for both of these functions.
 
-	''attr()'' functions are subs
 	''attr()'' functions are [=substitute an attr()|substituted=] at computed-value time.
 	If a declaration,
 	once all ''attr()'' functions are substituted in,