RDMA/mlx5: Remove unused IB_WR_REG_SIG_MR code

IB_WR_REG_SIG_MR is not needed after IB_WR_REG_MR_INTEGRITY
was used.

Signed-off-by: Israel Rukshin <israelr@mellanox.com>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>