[cssom-1] Remove incorrect note.

An @import rule always has an associated stylesheet, but there may not be (from
the author's perspective, at least), an empty cssRules list.

As mentioned in w3c#3452, network errors or other bits can make accessing the rules
throw a SecurityError.

Closes w3c#3452.

Author: emilio

cssom-1/Overview.bs

@@ -1793,8 +1793,7 @@ attribute of the associated <a>CSS style sheet</a>.
 
 The <dfn attribute for=CSSImportRule>styleSheet</dfn> attribute must return the associated <a>CSS style sheet</a>.
 
-Note: If loading of the style sheet fails its <a for=CSSStyleSheet>CSS rules</a>
-list is simply empty, i.e., an <code>@import</code> at-rule always has an associated <a>CSS style sheet</a>.
+Note: An <code>@import</code> at-rule always has an associated <a>CSS style sheet</a>.
 
 
 ### The {{CSSGroupingRule}} Interface ### {#the-cssgroupingrule-interface}