[jvm-packages] update checkstyle (#7335)

* [jvm-packages] update scalastyle

1. bump scalastyle-maven-plugin and maven-checkstyle-plugin to latest
2. remove unused imports

* fix code style check

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/Booster.java

@@ -1,5 +1,5 @@
 /*
- Copyright (c) 2014 by Contributors
+ Copyright (c) 2014,2021 by Contributors
 
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
@@ -436,7 +436,7 @@ public class Booster implements Serializable, KryoSerializable {
   }
 
   public String[] getModelDump(String[] featureNames, boolean withStats, String format)
-    throws XGBoostError {
+      throws XGBoostError {
     int statsFlag = 0;
     if (withStats) {
       statsFlag = 1;

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/RabitTracker.java

@@ -1,7 +1,5 @@
 package ml.dmlc.xgboost4j.java;
 
-
-
 import java.io.*;
 import java.util.HashMap;
 import java.util.Map;
@@ -80,7 +78,7 @@ public class RabitTracker implements IRabitTracker {
   }
 
   public RabitTracker(int numWorkers)
-    throws XGBoostError {
+      throws XGBoostError {
     if (numWorkers < 1) {
       throw new XGBoostError("numWorkers must be greater equal to one");
     }