[MINOR][CORE] Remove import scala.collection.Set in TaskSchedulerImpl

## What changes were proposed in this pull request?

I was playing with the scheduler and found this weird thing. In `TaskSchedulerImpl` we import `scala.collection.Set` without any reason. This is bad in practice, as it silently changes the actual class when we simply type `Set`, which by default should point to the immutable set.

This change only affects one method: `getExecutorsAliveOnHost`. I checked all the caller side and none of them need a general `Set` type.

## How was this patch tested?

N/A

Closes #24231 from cloud-fan/minor.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

core/src/main/scala/org/apache/spark/scheduler/TaskSchedulerImpl.scala

@@ -22,7 +22,6 @@ import java.util.{Locale, Timer, TimerTask}
 import java.util.concurrent.{ConcurrentHashMap, TimeUnit}
 import java.util.concurrent.atomic.AtomicLong
 
-import scala.collection.Set
 import scala.collection.mutable.{ArrayBuffer, BitSet, HashMap, HashSet}
 import scala.util.Random
 
@@ -829,8 +828,8 @@ private[spark] class TaskSchedulerImpl(
    * Get a snapshot of the currently blacklisted nodes for the entire application.  This is
    * thread-safe -- it can be called without a lock on the TaskScheduler.
    */
-  def nodeBlacklist(): scala.collection.immutable.Set[String] = {
-    blacklistTrackerOpt.map(_.nodeBlacklist()).getOrElse(scala.collection.immutable.Set())
+  def nodeBlacklist(): Set[String] = {
+    blacklistTrackerOpt.map(_.nodeBlacklist()).getOrElse(Set.empty)
   }
 
   // By default, rack is unknown