RDMA/rtrs: Remove unused field of rtrs_iu

list field is not used anywhere

Link: https://lore.kernel.org/r/20200930131407.6438-1-gi-oh.kim@clous.ionos.com
Signed-off-by: Gioh Kim <gi-oh.kim@cloud.ionos.com>
Acked-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>