QNSImageView - remove observer before dealloc

It can happen that NSMenuDidEndTrackingNotification comes after our imageCell
QNSImageView was deallocated (for example, mouse button is not released yet
when dealloc called). Remove soon-to-be-deallocated observer.

Change-Id: Ib155cc5f0b884c6b1fed0f986d12599096b582c6
Task-number: QTBUG-46425
Reviewed-by: Thierry Bastian <thierryb@filewave.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>