Remove visibility:collapse behavior, per ML discussion.

Author: tabatkins

css3-flexbox/Overview.html

@@ -477,13 +477,6 @@ <h3 id=flex-items><span class=secno>2.2. </span> Flexbox Items</h3>
     positioned and thus out of flow.</p>
   </div>
 
-  <p>Similar to table cells, flexbox items respond specially to the
-   &lsquo;<code class=css>collapse</code>&rsquo; value of the &lsquo;<code
-   class=property>visibility</code>&rsquo; property. When a flexbox item is
-   set to &lsquo;<code class=css>visibility: collapse;</code>&rsquo;, the
-   item does not generate a box at all (identical to setting &lsquo;<code
-   class=css>display: none;</code>&rsquo;).
-
   <h4 id=abspos-flexbox-items><span class=secno>2.2.1. </span> Positioning
    Absolutely Positioned Flexbox Items</h4>
 

css3-flexbox/Overview.src.html

@@ -191,8 +191,6 @@ <h3 id='flex-items'>
 		<p>Notice that block element "not-an-item6.3" is not a separate flexbox item, because it is contained inside an inline element which is being wrapped into an anonymous flex item.  Similarly, the block element "not-an-item1.5" is not a flexbox item, because it's absolutely positioned and thus out of flow.</p>
 	</div>
 
-	<p>Similar to table cells, flexbox items respond specially to the ''collapse'' value of the 'visibility' property.  When a flexbox item is set to ''visibility: collapse;'', the item does not generate a box at all (identical to setting ''display: none;'').</p>
-
 <h4 id='abspos-flexbox-items'>
 Positioning Absolutely Positioned Flexbox Items</h4>