C++: do not run GC when exiting.

This operation is quite costly if a lot of files are involved, and in
case of exiting Creator, it's also useless.

Change-Id: I97d178d47a3a2f6b214f7ebc45c871edd26b8286
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>