[SPARK-3305] Remove unused import from UI classes.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2200 from sarutak/SPARK-3305 and squashes the following commits:

3cbd6ee [Kousuke Saruta] Removed unused import from classes related to UI

core/src/main/scala/org/apache/spark/ui/exec/ExecutorsPage.scala

@@ -21,7 +21,6 @@ import javax.servlet.http.HttpServletRequest
 
 import scala.xml.Node
 
-import org.apache.spark.storage.StorageLevel
 import org.apache.spark.ui.{ToolTips, UIUtils, WebUIPage}
 import org.apache.spark.util.Utils
 

core/src/main/scala/org/apache/spark/ui/jobs/JobProgressListener.scala

@@ -17,7 +17,7 @@
 
 package org.apache.spark.ui.jobs
 
-import scala.collection.mutable.{HashMap, ListBuffer, Map}
+import scala.collection.mutable.{HashMap, ListBuffer}
 
 import org.apache.spark._
 import org.apache.spark.annotation.DeveloperApi