[fix](export) remove task from map finally (#51171)

### What problem does this PR solve?

Related PR: #47974

Problem Summary:

When cancel task, the task should be removed from `taskExecutorMap`,
otherwise, if `tryPublishTask()` failed when `addMemoryTask`, the task
will
be leaked in `taskExecutorMap`

Author: morningman

fe/fe-core/src/main/java/org/apache/doris/scheduler/manager/TransientTaskManager.java

@@ -63,7 +63,7 @@ public Long addMemoryTask(TransientTaskExecutor executor) throws JobException {
     }
 
     public void cancelMemoryTask(Long taskId) throws JobException {
-        TransientTaskExecutor transientTaskExecutor = taskExecutorMap.get(taskId);
+        TransientTaskExecutor transientTaskExecutor = taskExecutorMap.remove(taskId);
         if (transientTaskExecutor != null) {
             transientTaskExecutor.cancel();
         }