don't have to compare in step since it is done in init function

In `ActionInterval::initWithDuration()`, there is codes 
```cpp
// prevent division by 0
    // This comparison could be in step:, but it might decrease the performance
    // by 3% in heavy based action games.
    if (_duration == 0)
    {
        _duration = FLT_EPSILON;
    }
```

But it also compares in `ActionInterval::step()` too.