[css-font-loading-3] Remove unused callback dfn. Fixes w3c#5605

Author: tabatkins

css-font-loading-3/Overview.bs

@@ -453,8 +453,6 @@ The <code>FontFaceSet</code> Interface</h2>
 
 		enum FontFaceSetLoadStatus { "loading", "loaded" };
 
-		callback ForEachCallback = undefined (FontFace font, long index, FontFaceSet self);
-
 		[Exposed=(Window,Worker)]
 		interface FontFaceSet : EventTarget {
 			constructor(sequence<FontFace> initialFaces);
@@ -994,7 +992,7 @@ Interaction with CSS Font Loading and Matching</h3>
 -->
 
 <h2 id='font-face-source'>
-The <code>FontFaceSource</code> mixin</h2>
+The <code>FontFaceSource</code> Mixin</h2>
 
 	<pre class='idl'>
 		interface mixin FontFaceSource {