delete no use var

Author: wangfei <wangfei_hello@126.com>

Closes #613 from scwf/masterIndex and squashes the following commits:

1463056 [wangfei] delete no use var: masterIndex

core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala

@@ -71,8 +71,6 @@ private[spark] class Worker(
   // TTL for app folders/data;  after TTL expires it will be cleaned up
   val APP_DATA_RETENTION_SECS = conf.getLong("spark.worker.cleanup.appDataTtl", 7 * 24 * 3600)
 
-  // Index into masterUrls that we're currently trying to register with.
-  var masterIndex = 0
 
   val masterLock: Object = new Object()
   var master: ActorSelection = null