net/mlx5: Remove vport QoS enabled flag

Remove the `enabled` flag from the `vport->qos` struct, as QoS now
relies solely on the `sched_node` pointer to determine whether QoS
features are in use.

Currently, the vport `qos` struct consists only of the `sched_node`,
introducing an unnecessary two-level reference. However, the qos struct
is retained as it will be extended in future patches to support new QoS
features.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>