I believe that these words are a reference to Christ's final words on the cross (you can read about Tolstoy's struggle with faith here on eNotes to see how this is part of his theme of death in this novel). Before he died, one of the last things Christ said was, "It is finished." Like in the novel, this has a double meaning. Christ's life, like Ivan's, was "finished" at this point, but Christ was also referring to his life's work. Christians believe that Christ came to earth for the very purpose of dying for the sins of mankind, so when he said "It is finished" he meant his life's work was finished. When these words are said in Tolstoy's novel, it not only means that Ivan's life is over, but also that death has no more control of him. All his life, he feared death, but now that he is so close to death, he has an epiphany and realizes what his life was all about. So, what "is no more" is the pain, not only the physical pain, but the mental pain of living with the fear of death that obsessed Ivan throughout his life after he became ill.
Read about it here on eNotes.