[SPARK-8981][CORE][FOLLOW-UP] Clean up MDC properties after running a task

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

This PR is a followup of #26624. This PR cleans up MDC properties if the original value is empty.
Besides, this PR adds a warning and ignore the value when the user tries to override the value of `taskName`.

### Why are the changes needed?

Before this PR, running the following jobs:

```
sc.setLocalProperty("mdc.my", "ABC")
sc.parallelize(1 to 100).count()
sc.setLocalProperty("mdc.my", null)
sc.parallelize(1 to 100).count()
```

there's still MDC value "ABC" in the log of the second count job even if we've unset the value.

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

Yes, user will 1) no longer see the MDC values after unsetting the value; 2) see a warning if he/she tries to override the value of `taskName`.

### How was this patch tested?

Tested Manaually.

Closes #28756 from Ngone51/followup-8981.

Authored-by: yi.wu <yi.wu@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: Ngone51

core/src/main/scala/org/apache/spark/executor/Executor.scala

@@ -401,9 +401,7 @@ private[spark] class Executor(
     }
 
     override def run(): Unit = {
-
       setMDCForTask(taskName, mdcProperties)
-
       threadId = Thread.currentThread.getId
       Thread.currentThread.setName(threadName)
       val threadMXBean = ManagementFactory.getThreadMXBean
@@ -703,11 +701,11 @@ private[spark] class Executor(
   }
 
   private def setMDCForTask(taskName: String, mdc: Seq[(String, String)]): Unit = {
+    // make sure we run the task with the user-specified mdc properties only
+    MDC.clear()
+    mdc.foreach { case (key, value) => MDC.put(key, value) }
+    // avoid overriding the takName by the user
     MDC.put("taskName", taskName)
-
-    mdc.foreach { case (key, value) =>
-      MDC.put(key, value)
-    }
   }
 
   /**
@@ -750,9 +748,7 @@ private[spark] class Executor(
     private[this] val takeThreadDump: Boolean = conf.get(TASK_REAPER_THREAD_DUMP)
 
     override def run(): Unit = {
-
       setMDCForTask(taskRunner.taskName, taskRunner.mdcProperties)
-
       val startTimeNs = System.nanoTime()
       def elapsedTimeNs = System.nanoTime() - startTimeNs
       def timeoutExceeded(): Boolean = killTimeoutNs > 0 && elapsedTimeNs > killTimeoutNs