Removed lingering reference to <image-combination> (referring to the cross-fade() function that was punted to level 4)

Author: tabatkins

css3-images/Overview.html

@@ -362,7 +362,7 @@ <h2 id=image><span class=secno>3. </span> Image Values: the &lt;image> type</h2>
   <p>The &lt;image> value type denotes a 2D image. It is defined as
 
   <pre class=prod><dfn
-   id=ltimage>&lt;image></dfn> = &lt;url> | &lt;image-list> | &lt;element-reference> | &lt;image-combination> | &lt;gradient></pre>
+   id=ltimage>&lt;image></dfn> = &lt;url> | &lt;image-list> | &lt;element-reference>  | &lt;gradient></pre>
 
   <p>Image values can be used in many CSS properties, including the
    &lsquo;<code class=property>background-image</code>&rsquo;, &lsquo;<code

css3-images/Overview.src.html

@@ -138,7 +138,7 @@ <h2 id="image">
 
 	<p>The &lt;image> value type denotes a 2D image. It is defined as
 
-	<pre class="prod"><dfn>&lt;image></dfn> = &lt;url> | &lt;image-list> | &lt;element-reference> | &lt;image-combination> | &lt;gradient></pre>
+	<pre class="prod"><dfn>&lt;image></dfn> = &lt;url> | &lt;image-list> | &lt;element-reference>  | &lt;gradient></pre>
 
 	<p>Image values can be used in many CSS properties, including the
 	'background-image', 'list-style-image', 'cursor' properties [[!CSS21]].