[resize-observer] Removed line in spec which states you can disconnect() an in use resizeObserver - fixes w3c#3662

Author: Greg Whitworth

resize-observer-1/Overview.bs

@@ -211,8 +211,6 @@ interface ResizeObserver {
 
         2. Clear the {{ResizeObserver/activeTargets}} list.
 
-        3. Remove |this| from {{Document}}.<var>resizeObservers</var> slot.
-
 </div>
 
 <h3 id="resize-observer-callback">ResizeObserverCallback</h3>