RDMA/rxe: Remove an unused member from struct rxe_mr

RDMA/rxe: Remove an unused member from struct rxe_mr

Commit 1e75550648da ("Revert "RDMA/rxe: Create duplicate mapping tables for
FMRs"") brought back the member 'va' to struct rxe_mr. However, it is
actually used by nobody and thus can be removed.

Fixes: 1e75550648da ("Revert "RDMA/rxe: Create duplicate mapping tables for FMRs"")
Link: https://lore.kernel.org/r/20220829012335.1212697-1-matsuda-daisuke@fujitsu.com
Signed-off-by: Daisuke Matsuda <matsuda-daisuke@fujitsu.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>