Remove unnecessary constructors from exception class

exercises/triangle/src/example/java/TriangleException.java

-public class TriangleException extends Exception {
+class TriangleException extends Exception {
 
-    public TriangleException() {
-    }
 }

exercises/triangle/src/main/java/TriangleException.java

-public class TriangleException extends Exception {
+class TriangleException extends Exception {
 
-    public TriangleException() {
-    }
 }