RDMA/mlx5: Delete not needed GSI QP signal QP type

GSI QP doesn't need signal QP type because it is initialized statically to
zero, which is IB_SIGNAL_ALL_WR also wr->send_flags isn't set too.  This
means that the GSI QP signal QP type can be removed.

Link: https://lore.kernel.org/r/20200926102450.2966017-5-leon@kernel.org
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>