Remove definition of class strings of default iterator objects (#501)

Per consensus reached in #483, class strings should only exist on
iterator prototype objects.

index.bs

@@ -11909,9 +11909,10 @@ use standard ECMAScript Array iterator objects.
 When a [=default iterator object=] is first created,
 its index is set to 0.
 
-The [=class string=] of a [=default iterator object=] for a given [=interface=]
-is the result of concatenating the [=identifier=] of the [=interface=]
-and the string "<code> Iterator</code>".
+[=Default iterator objects=] do not have [=class strings=]; when <code
+class="idl">Object.prototype.toString()</code> is called on a [=default
+iterator object=] of a given [=interface=], the [=class string=] of the
+[=iterator prototype object=] of that [=interface=] is used.
 
 
 <h5 id="es-iterator-prototype-object">Iterator prototype object</h5>