[SPARK-3518] Remove wasted statement in JsonProtocol

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2380 from sarutak/SPARK-3518 and squashes the following commits:

8a1464e [Kousuke Saruta] Replaced a variable with simple field reference
c660fbc [Kousuke Saruta] Removed useless statement in JsonProtocol.scala

(cherry picked from commit e59fac1f)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>

core/src/main/scala/org/apache/spark/util/JsonProtocol.scala

@@ -202,7 +202,6 @@ private[spark] object JsonProtocol {
   }
 
   def taskInfoToJson(taskInfo: TaskInfo): JValue = {
-    val accumUpdateMap = taskInfo.accumulables
     ("Task ID" -> taskInfo.taskId) ~
     ("Index" -> taskInfo.index) ~
     ("Attempt" -> taskInfo.attempt) ~