it depends on whether you installed them in the mods folder or in the update folder. if in the mods folder, just delete it (CAREFUL doing so you will uninstall all mods, such as graphic mod, mod for sound etc.)
If, on the other hand, you have installed them in the update, I advise you to reinstall the game (it is only 97 GB 🙂
-If, however, you want to play online and you have installed the cars in the mod folder you can do it without deleting them.