[SPARK-15045] [CORE] Remove dead code in TaskMemoryManager.cleanUpAllAllocatedMemory for pageTable

## What changes were proposed in this pull request?

Removed the DeadCode as suggested.

Author: Abhinav Gupta <abhi.951990@gmail.com>

Closes #12829 from abhi951990/master.

core/src/main/java/org/apache/spark/memory/TaskMemoryManager.java

@@ -379,7 +379,6 @@ public class TaskMemoryManager {
    */
   public long cleanUpAllAllocatedMemory() {
     synchronized (this) {
-      Arrays.fill(pageTable, null);
       for (MemoryConsumer c: consumers) {
         if (c != null && c.getUsed() > 0) {
           // In case of failed task, it's normal to see leaked memory
@@ -387,14 +386,16 @@ public class TaskMemoryManager {
         }
       }
       consumers.clear();
-    }
 
-    for (MemoryBlock page : pageTable) {
-      if (page != null) {
-        memoryManager.tungstenMemoryAllocator().free(page);
+      for (MemoryBlock page : pageTable) {
+        if (page != null) {
+          logger.warn("leak a page: " + page + " in task " + taskAttemptId);
+          memoryManager.tungstenMemoryAllocator().free(page);
+        }
       }
+      Arrays.fill(pageTable, null);
     }
-    Arrays.fill(pageTable, null);
+
 
     // release the memory that is not used by any consumer.
     memoryManager.releaseExecutionMemory(acquiredButNotUsed, taskAttemptId, tungstenMemoryMode);