remove final from class declaration in secret-handshake exercise

exercises/secret-handshake/src/main/java/HandshakeCalculator.java

 import java.util.List;
 
-final class HandshakeCalculator {
+class HandshakeCalculator {
 
     List<Signal> calculateHandshake(int number) {
         throw new UnsupportedOperationException("Delete this statement and write your own implementation.");