RDMA/rxe: Remove reliable datagram support

The rdma_rxe driver does not actually support the reliable datagram
transport but contains a variable with RD opcodes in driver code.  And
this variable is never used. So remove it.

Link: https://lore.kernel.org/r/20221112023537.432912-1-yanjun.zhu@intel.com


Signed-off-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>