A chicken would need to eat unrealistic amounts of apple seeds in a short time before the cyanide caused any realistic harm, they actually have a high resistance to cyanide...
To put it in perspective the LD50 of cyanide in domestic fowl is 21 mg/kg... In layman's terms that means if you gave 100 chickens 21 mg of cyanide per 1 kg of body weight 50% would pass away...
An apple seed contains about 0.5mg of cyanide, thus you need about 42 seeds to get to the lethal dose range for a 1kg bird, since most standards are about 1.5kg or more, that would mean a single chicken would need to eat about 63 seed or 6 - 7 apples worth of seeds all in a very short period of time to approach the lethal does...
A chickens digestive track is only so big, you can bet when you offer apples they are filling that up with apple, not the seeds
Not saying it's a bad idea to remove apple seeds, but in the end it's not likely to cause any harm if you don't unless that is all you feed them 24/7...