Remove elem property from event handle

As discussed with @dmethvin on jQuery Russia, elem is removed from the event handle stored in data_priv.

1) This property helped to fix memory leaks in IE<9. Not needed in 2.0 => removal is good.

2) If an element with bound events is removed without cleanData (e.g with native DOM ops), then it will remain in memory with the whole DOM subtree because elemData references it. 
After the patch which removes the reference, the memory can be freed. This will help to shrink memory leaks in such cases making them barely noticeable.