remove final from class declaration in robot-simulator exercise

exercises/robot-simulator/src/main/java/GridPosition.java

@@ -1,4 +1,4 @@
-final class GridPosition {
+class GridPosition {
 
     final int x;
 

exercises/robot-simulator/src/main/java/Robot.java

@@ -1,4 +1,4 @@
-final class Robot {
+class Robot {