It's clearly not just leftover code, the tires themselves are modeled in full detail, and not only that, they were updated in the most recent patch. It leads me to believe that at minimum, PD had planned to use them, but weren't able to get rain/wet surfaces to run smoothly, and/or didn't have a snow track ready at launch.
The fact that there's now models for the rain tires makes me very curious about PD's internal developments. I don't actually recall their statements around rain, but I wonder if static weather options are on the way. It'd seem like they'd most easily be able to cram in some rain particles and altered surface textures on the existing "overcast" conditions on many of the tracks. I'd bet the holdup (if it is in fact a thing thats in the works) is making the PS4 hardware deal with added particle effects that would naturally come with wet weather.