Checkstyle fix

Author: yuvashreenarayanan3

src/main/java/com/thealgorithms/backtracking/ArrayCombination.java

@@ -2,7 +2,6 @@
 
 import java.util.ArrayList;
 import java.util.List;
-import java.util.TreeSet;
 
 /**
  * Finds all permutations of 1...n of length k

src/test/java/com/thealgorithms/backtracking/ArrayCombinationTest.java

@@ -1,10 +1,9 @@
 package com.thealgorithms.backtracking;
 
-
 import java.util.ArrayList;
 import java.util.List;
 
-import org.junit.jupiter.api.Test; 
+import org.junit.jupiter.api.Test;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
 public class ArrayCombinationTest {
@@ -46,4 +45,4 @@ public void testSingleElementCombinations() {
 
         assertEquals(expected, actual);
     }
-}
+}