Reports have suggested Smith could decide to sign a contract extension with the club.
“I’ve made my decision. It was an awkward one but it’s the right thing for me and the club,” Smith said.
“The last time, I maybe hung around for too long. This time, I didn’t expect to be back for as long.
“The job needs a younger person to give an edge, with continued motivation to overcome the problems at the club.
“It’s best for both parties – I will not change my mind.”
