[css-variables] Remove note talking about custom property containing only whitespace, as that whitespace is now trimmed away, per Syntax. Relates to w3c#774.

Author: tabatkins

css-variables-1/Overview.bs

@@ -160,12 +160,6 @@ Custom Property Value Syntax</h3>
 	does not prevent ''!important'' from being used,
 	as the ''!important'' is removed before syntax checking happens.
 
-	Note: While <<declaration-value>> must represent at least one token,
-	that one token may be whitespace.
-	This implies that ''--foo: ;'' is valid,
-	and the corresponding ''var(--foo)'' call would have a single space as its substitution value,
-	but ''--foo:;'' is invalid.
-
 	<div class='example'>
 		For example, the following is a valid custom property: