Fix "Utility classes should not have a public or default constructor."

Author: alxkm

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

@@ -7,6 +7,9 @@
  * Class generates all subsequences for a given list of elements using backtracking
  */
 public class Subsequence {
+    private Subsequence() {
+    }
+
     /**
      * Find all subsequences of given list using backtracking
      *