remove placement object on scene closing to address issue that leads to abandoned objects

[chr-ber]

I kept having more and more abandoned placement objects when switching between scenes in paint mode.

This should no longer occure when deleting the placement object before scene save. Originally it was getting deleted also when editomode was swtiched to none but I have found that this might be redundant now.

The objects also get deleted when:

• Editor looses focus (OnLostFocus), is deleting the object most of the time.
• Editor is getting closed (OnDestroy)