Fix flickering popover animations

Closes #4138

- Removed JS animation to close in favour of CSS animations
Here's what I'm hearing if my above was correct,

| open | closing | fade-out |
--------------------------------------
|  1   |   1     |    1     |
|  1   |   0     |    0     |
|  0   |   1     |    1     |
|  0   |   0     |    1     |

In which case,
```js
fadeIn: Ember.computed('open', 'closing', function () {
  return this.get('open') && !this.get('closing');
})
```