net/mlx5_core/health: Remove deprecated create_singlethread_workqueue

net/mlx5_core/health: Remove deprecated create_singlethread_workqueue

The workqueue health->wq was used as per device private health thread.
This was done to perform delayed work.

The workqueue has a single workitem(&health->work) and
hence doesn't require ordering. It is involved in handling the health of
the device and is not being used on a memory reclaim path.
Hence, the singlethreaded workqueue has been replaced with the use of
system_wq.

Work item has been flushed in mlx5_health_cleanup() to ensure that
there are no pending tasks while disconnecting the driver.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>