Code cleanup

- Remove unused assignment to null;
- Remove useless casting to String
- Remove useless checkState because ghostdriverpath is never null at this point
Look at the initial code:
```java
        if (ghostdriverpath != null) {
            checkState(ghostdriverpath != null,
                    "The path to the driver executable must be ...", ...);
```

Inside the "if" we know that `ghostdriverpath` is NOT null. In this case `checkState` call does **nothing**.