To my knowledge, dlc package assets are not handled the same as DLS installed assets in your local database. There is only a pointer record that references the assets as located in the tzarc files. The assets that are part of the dlc package remain inside the files in the packages folder. So you can't delete individual assets. You can uninstall the dlc packages from the Trainz Content interface. At least, that is how I interpret the behavior they show.