IB/iser: Get rid of struct iser_rdma_regd

This struct members other than struct iser_mem_reg are unused,
so remove it altogether.

This patch does not change any functionality.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Adir Lev <adirl@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>