[SPARK-48034][TESTS] NullPointerException in MapStatusesSerDeserBench…

…mark

### What changes were proposed in this pull request?

This PR fixes an NPE in MapStatusesSerDeserBenchmark. The cause is that we try to stop the tracker twice.

```
3197java.lang.NullPointerException: Cannot invoke "org.apache.spark.rpc.RpcEndpointRef.askSync(Object, scala.reflect.ClassTag)" because the return value of "org.apache.spark.MapOutputTracker.trackerEndpoint()" is null
3198	at org.apache.spark.MapOutputTracker.askTracker(MapOutputTracker.scala:541)
3199	at org.apache.spark.MapOutputTracker.sendTracker(MapOutputTracker.scala:551)
3200	at org.apache.spark.MapOutputTrackerMaster.stop(MapOutputTracker.scala:1242)
3201	at org.apache.spark.SparkEnv.stop(SparkEnv.scala:112)
3202	at org.apache.spark.SparkContext.$anonfun$stop$25(SparkContext.scala:2354)
3203	at org.apache.spark.util.Utils$.tryLogNonFatalError(Utils.scala:1294)
3204	at org.apache.spark.SparkContext.stop(SparkContext.scala:2354)
3205	at org.apache.spark.SparkContext.stop(SparkContext.scala:2259)
3206	at org.apache.spark.MapStatusesSerDeserBenchmark$.afterAll(MapStatusesSerDeserBenchmark.scala:128)
3207	at org.apache.spark.benchmark.BenchmarkBase.main(BenchmarkBase.scala:80)
3208	at org.apache.spark.MapStatusesSerDeserBenchmark.main(MapStatusesSerDeserBenchmark.scala)
3209	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
3210	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
3211	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
3212	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
3213	at org.apache.spark.benchmark.Benchmarks$.$anonfun$main$7(Benchmarks.scala:128)
3214	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
3215	at org.apache.spark.benchmark.Benchmarks$.main(Benchmarks.scala:91)
3216	at org.apache.spark.benchmark.Benchmarks.main(Benchmarks.scala)
```
### Why are the changes needed?

test bugfix

### Does this PR introduce _any_ user-facing change?

no

### How was this patch tested?

manually

### Was this patch authored or co-authored using generative AI tooling?
no

Closes #46270 from yaooqinn/SPARK-48034.

Authored-by: Kent Yao <yao@apache.org>
Signed-off-by: Kent Yao <yao@apache.org>
(cherry picked from commit 59d5946)
Signed-off-by: Kent Yao <yao@apache.org>

core/src/test/scala/org/apache/spark/MapStatusesSerDeserBenchmark.scala

@@ -123,7 +123,6 @@ object MapStatusesSerDeserBenchmark extends BenchmarkBase {
   }
 
   override def afterAll(): Unit = {
-    tracker.stop()
     if (sc != null) {
       sc.stop()
     }