I figured it out. Go to this folder: Automobilista 2\Vehicles\Textures\CustomLiveries\Overrides. Find the car with the custom liveries you want to remove, and delete or cut the XML file with the name of the car. For example: in the cart_reynard_98i_ford folder, there will be a file with the same name. Cut that file. Leave the other file (cart_reynard_98i_ford_dist). This is just a template and instructions for adding custom skins.