[SPARK-45776][CORE] Remove the defensive null check for MapOutputTrackerMaster#unregisterShuffle added in SPARK-39553

### What changes were proposed in this pull request?
This pr Remove the defensive null check for `MapOutputTrackerMaster#unregisterShuffle` added in SPARK-39553.

### Why are the changes needed?
scala/bug#12613 has been fixed in Scala 2.13.9.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Existing test like `SPARK-39553: Multi-thread unregister shuffle shouldn't throw NPE` in `MapOutputTrackerSuite`

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

Closes #43644 from LuciferYang/remove-39553-null-check.

Lead-authored-by: yangjie01 <[email protected]>
Co-authored-by: YangJie <[email protected]>
Signed-off-by: Kent Yao <[email protected]>

Author: LuciferYang

core/src/main/scala/org/apache/spark/MapOutputTracker.scala

@@ -906,12 +906,8 @@ private[spark] class MapOutputTrackerMaster(
   /** Unregister shuffle data */
   def unregisterShuffle(shuffleId: Int): Unit = {
     shuffleStatuses.remove(shuffleId).foreach { shuffleStatus =>
-      // SPARK-39553: Add protection for Scala 2.13 due to https://github.com/scala/bug/issues/12613
-      // We should revert this if Scala 2.13 solves this issue.
-      if (shuffleStatus != null) {
-        shuffleStatus.invalidateSerializedMapOutputStatusCache()
-        shuffleStatus.invalidateSerializedMergeOutputStatusCache()
-      }
+      shuffleStatus.invalidateSerializedMapOutputStatusCache()
+      shuffleStatus.invalidateSerializedMergeOutputStatusCache()
     }
   }