There seems to be some confusion about the religion of Charles Taze Russell. While the Jehovah's Witnesses are associated with him, and he founded the Bible Student movement, which they evolved from, he was not a Jehovah's Witness himself. It was after his death that the movement reorganized and formally adopted the name "Jehovah's Witnesses."
During Charles Taze Russell's time, it was common for crosses to be used as symbols in various Christian denominations, but the Bible Student movement moved away from using the cross as a religious symbol. Therefore, if the cross was removed from Charles Taze Russell's grave, it may have been because the Bible Students did not adhere to the traditional Christian use of the cross.