Continue feeding it until it is gone. It will do absolutely no harm. You can mix it with the Grower if you wish to reduce the protein content and are lucky enough to be able to find Grower. I like to switch around 6 weeks if I have the option but I usually have to stick with the combined Starter/Grower because that often is all I have available. I always continue to feed what I have left, usually mixing it when I switch to ease the transition, but don't be surprised if they pick out one instead of the other. When mine do that, I don't give them any more until they clean their plate. They do clean it up eventually. I don't like to have old feed in their feeder, It loses potency and may go moldy.
Since your chicks were not vaccinated for cocci, you have not done them any harm at all feeding the mixed medicated feed. The medicine in the medicated is probably Amprol or an Amprolium product which inhibits the reproduction of the protazoa that causes cocci in the intestines of the chick. It does not completely stop the reproduction so the chick can still develop the immunity it needs. A smaller amount of the Amprol just means less restrictions on the reproduction of that protazoa if it is present. If you raised them in a fairly dry brooder, cocci was probably never a threat anyway.