The 29-year-old’s current contract is set to expire this summer.
“Robert has been a great servant to the club and we would like to thank him for all his efforts,” sporting and technical director Dan Ashworth told the club’s official website.
“He scored some important goals for us and has been very professional in everything he has done.
“However, on our return to the Premier League we have to try and get the balance of the squad right and we feel we are best served concentrating our available resources into other areas of the squad.
“We also have to be aware of the new Premier League rules regarding the number of homegrown players that must be included in the squad.”
