RDMA/rxe: Remove reliable datagram support

The rdma_rxe driver does not actually support the reliable datagram
transport but contains two references to RD opcodes in driver code.  This
commit removes these references to RD transport opcodes which are never
used.

Link: https://lore.kernel.org/r/cce0f07d-25fc-5880-69e7-001d951750b7@gmail.com


Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>