remove reference to previously-removed margin-box value of mask-clip (w3c#535)

Author: alkorlos

css-masking-1/Overview.bs

@@ -656,7 +656,7 @@ Values have the following meanings:
     :: The painted content is not restricted (not clipped).
 </dl>
 
-For SVG elements without associated CSS layout box, the <a>used value</a>s for ''mask-clip/content-box'' and ''mask-clip/padding-box'' compute to ''mask-clip/fill-box'' and for ''mask-clip/border-box'' and ''mask-clip/margin-box'' compute to ''mask-clip/stroke-box''.
+For SVG elements without associated CSS layout box, the <a>used value</a>s for ''mask-clip/content-box'' and ''mask-clip/padding-box'' compute to ''mask-clip/fill-box'' and for ''mask-clip/border-box'' compute to ''mask-clip/stroke-box''.
 
 For elements with associated CSS layout box, the <a>used value</a>s for ''mask-clip/fill-box'' compute to ''mask-clip/content-box'' and for ''mask-clip/stroke-box'' and ''mask-clip/view-box'' compute to ''mask-clip/border-box''.