You cannot disappear a car objective nor in/during a cutscene nor outside of it, but there is a way that you can disappear a car objective in/during a cutscene. After the objective is completed, a cutscene will appear. To not notice the car soon, put a secondary cutscene about something to give time. -» Make an actor close to the car objective and choose the animation ''sit in closest car'' and make a route (the most recommended is far away as possible from the place).
A mission with this technique: http://dyom.gtagames.nl/show/24634
Enjoy!