Remove struct rxe_dev mc_grp_pool field. This field is no longer used.

Link: https://lore.kernel.org/r/20220407184849.14359-1-rpearsonhpe@gmail.comSigned-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>