fix pmd

Author: Samuel Facchinello

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

@@ -2,7 +2,6 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNull;
-import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.util.List;
 import java.util.TreeSet;