AVFoundation: fix some controls not being correctly destroyed on iOS.

This patch also makes sure AVF video layers are removed from their
parent layer when their corresponding Qt video outputs are destroyed.

Task-number: QTBUG-39385
Change-Id: I164cd0da7084f84c0473ed3e396e734acce2a22e
Reviewed-by: Andy Nichols <andy.nichols@digia.com>