Draggable: $.ui.ddmanager.current release on draggable _destroy

Fixes: #10667
> Well, I'm all confused now... :)

That's a bit disconcerting. I can't tell how familiar you are with this code or how much you researched this code or how much testing you've done with your changes. You started with comparing jQuery objects, which is never a good check because you can very easily have essentially equivalent objects that aren't equal. I suggested checking the actual elements, but the code got quite long and when I looked at how `$.ui.ddmanager.current` is actually used, it seemed like the check could be simplified quite a bit.

> Can it conclude with
> 
> if ($.ui.ddmanager && $.ui.ddmanager.current === this) {
>   $.ui.ddmanager.current = null;
> }

To be honest, I expected you to do a fair amount of this work as the person initiating the change. I assume this is correct, but I'm not familiar with this code.

> I only need that in _destroy of Draggable and Droppable. Can you help here
> and push this change please?

I've been trying to help, but this is extremely low priority for us since all interactions are in the process of being rewritten.