RDMA/rxe: Remove unneeded pool->state

rxe_pool.c uses the field pool->state to mark a pool as invalid when it is
shut down and checks it in several pool APIs to verify that the pool has
not been shut down. This is unneeded because the pools are not marked
invalid unless the entire driver is being removed at which point no
functional APIs should or could be executing. This patch removes this
field and associated code.

Link: https://lore.kernel.org/r/20210125211641.2694-6-rpearson@hpe.com
Suggested-by:  <zyjzyj2000@gmail.c>
Signed-off-by: Bob Pearson <rpearson@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>