Revert "fix spark tests on machines with many cores (dmlc#4634)"

This reverts commit e3906de.

jvm-packages/xgboost4j-spark/src/test/scala/ml/dmlc/xgboost4j/scala/spark/PerTest.scala

@@ -19,24 +19,23 @@ package ml.dmlc.xgboost4j.scala.spark
 import java.io.File
 
 import ml.dmlc.xgboost4j.{LabeledPoint => XGBLabeledPoint}
+
 import org.apache.spark.{SparkConf, SparkContext, TaskFailedListener}
 import org.apache.spark.sql._
 import org.scalatest.{BeforeAndAfterEach, FunSuite}
-
-import scala.math.min
 import scala.util.Random
 
 trait PerTest extends BeforeAndAfterEach { self: FunSuite =>
 
-  protected val numWorkers: Int = min(Runtime.getRuntime.availableProcessors(), 4)
+  protected val numWorkers: Int = Runtime.getRuntime.availableProcessors()
 
   @transient private var currentSession: SparkSession = _
 
   def ss: SparkSession = getOrCreateSession
   implicit def sc: SparkContext = ss.sparkContext
 
   protected def sparkSessionBuilder: SparkSession.Builder = SparkSession.builder()
-      .master(s"local[${numWorkers}]")
+      .master("local[*]")
       .appName("XGBoostSuite")
       .config("spark.ui.enabled", false)
       .config("spark.driver.memory", "512m")

jvm-packages/xgboost4j-spark/src/test/scala/org/apache/spark/SparkParallelismTrackerSuite.scala

@@ -18,17 +18,16 @@ package org.apache.spark
 
 import org.scalatest.FunSuite
 import _root_.ml.dmlc.xgboost4j.scala.spark.PerTest
+
 import org.apache.spark.rdd.RDD
 import org.apache.spark.sql.SparkSession
 
-import scala.math.min
-
 class SparkParallelismTrackerSuite extends FunSuite with PerTest {
 
-  val numParallelism: Int = min(Runtime.getRuntime.availableProcessors(), 4)
+  val numParallelism: Int = Runtime.getRuntime.availableProcessors()
 
   override protected def sparkSessionBuilder: SparkSession.Builder = SparkSession.builder()
-    .master(s"local[${numParallelism}]")
+    .master("local[*]")
     .appName("XGBoostSuite")
     .config("spark.ui.enabled", true)
     .config("spark.driver.memory", "512m")