selftests: mlxsw: qos_lib: Drop __mlnx_qos

Now that the two users of this helper have been converted to iproute2 dcb,
it is not necessary anymore. Drop it.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>