net/mlx5: Remove unused mlx5dr_domain_sync

net/mlx5: Remove unused mlx5dr_domain_sync

mlx5dr_domain_sync() was added in 2019 by
commit 70605ea545e8 ("net/mlx5: DR, Expose APIs for direct rule managing")
but hasn't been used.

Remove it.

mlx5dr_domain_sync() was the only user of
mlx5dr_send_ring_force_drain().
Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Link: https://patch.msgid.link/20250203185958.204794-1-linux@treblig.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>