Remove from hover list on destruction

When hoverEnabled is unset the item remains in a hovered state until the
mouse leaves. In this circumstance, the item could be destroyed without
cleaning up the reference in the hovered items list.

To catch this and similar circumstances, the item is removed from the
list when destroyed, regardless of hoverEnabled status.

Task-number: QTBUG-30962
Change-Id: I982ab8c9e1077fecc885fd166d70bbbbbe8fc179
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Liang Qi <liang.qi@digia.com>