FIx leaky listeners

This is for #9418 
it's removing the event listener from 2 different elements though so you still need 2 calls.

I originally had written this as:

```javascript
   $scrollElement.off(".scroller-shadow");         
   $displayElement.off(".scroller-shadow");
```

But revised it after I saw what we were doing in `removeScrollerShadow()`.  Either one will work but wanted to be consistent and thought that what was in `removeScrollerShadow()` was more targeted and less risky.