Fix flickering popover animations

Closes #4138

- Removed JS animation to close in favour of CSS animations
I tried moving this into a computed property:

```
    //Managed the toggle between the fade-in and fade-out classes
    fadeClass: Ember.computed('closing', 'isOpen', function () {
        if (this.get('closing')) {
            return 'fade-out';
        }

        if (this.get('isOpen')) {
            return 'fade-in';
        }

        return 'fade-out';
    }),
```

I'd say that it's not worth moving it into that - it's more difficult to understand than the current implementation.