[SPARK-21941] Stop storing unused attemptId in SQLTaskMetrics

## What changes were proposed in this pull request?

In a driver heap dump containing 390,105 instances of SQLTaskMetrics this
would have saved me approximately 3.2MB of memory.

Since we're not getting any benefit from storing this unused value, let's
eliminate it until a future PR makes use of it.

## How was this patch tested?

Existing unit tests

Author: Andrew Ash <[email protected]>

Closes apache#19153 from ash211/aash/trim-sql-listener.

sql/core/src/main/scala/org/apache/spark/sql/execution/ui/SQLListener.scala

@@ -255,10 +255,8 @@ class SQLListener(conf: SparkConf) extends SparkListener with Logging {
                 // heartbeat reports
               }
             case None =>
-              // TODO Now just set attemptId to 0. Should fix here when we can get the attempt
-              // id from SparkListenerExecutorMetricsUpdate
               stageMetrics.taskIdToMetricUpdates(taskId) = new SQLTaskMetrics(
-                  attemptId = 0, finished = finishTask, accumulatorUpdates)
+                  finished = finishTask, accumulatorUpdates)
           }
         }
       case None =>
@@ -478,10 +476,11 @@ private[ui] class SQLStageMetrics(
     val stageAttemptId: Long,
     val taskIdToMetricUpdates: mutable.HashMap[Long, SQLTaskMetrics] = mutable.HashMap.empty)
 
+
+// TODO Should add attemptId here when we can get it from SparkListenerExecutorMetricsUpdate
 /**
  * Store all accumulatorUpdates for a Spark task.
  */
 private[ui] class SQLTaskMetrics(
-    val attemptId: Long, // TODO not used yet
     var finished: Boolean,
     var accumulatorUpdates: Seq[(Long, Any)])