RDMA/rtrs-srv: Kill rtrs_srv_change_state_get_old

This function isn't needed since no caller checks the old_state of sess.

Link: https://lore.kernel.org/r/20201023074353.21946-11-jinpu.wang@cloud.ionos.com
Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>