Remove comment about ordering.

While ordering is nice to have, it's hard to guarantee that, even with
ordering, the listing won't change between two client requests (and
thus end up with different info when the UI applies the paging
parameters). So don't make it a requirement (even if an informal one).

Author: Marcelo Vanzin

core/src/main/scala/org/apache/spark/deploy/history/HistoryServer.scala

@@ -269,8 +269,6 @@ private[spark] abstract class ApplicationHistoryProvider {
    * This method should return a list of applications available for the history server to
    * show.
    *
-   * The listing is assumed to be in descending end time order.
-   *
    * @return List of all know applications.
    */
   def getListing(): Seq[ApplicationHistoryInfo]