Fix MODx.Ajax.request ot handle multiple concurrent requests

### What does it do ?

Remove "listeners" in `MODx.Ajax` in favor of directly executing the callback, using the same "listener" syntax to be BC.

### Why is it needed ?

`MODx.Ajax` currently rely on listeners to perform callbacks on some events.
Each new `MODx.Ajax.request` resets previously set listeners, resulting in errors when doing concurrent requests (last added listeners being trigger for each request)

### To do before merge

- [ ] Check on as much browsers as possible (yeah, looking at you IE), since this has only been tested on Chrome/Linux

### Related issue(s)/PR(s)

* #2821