net/mlx5e: Remove leftovers from old XSK queues enumeration

Before the cited commit, for N channels, a dedicated set of N queues was
created to support XSK, in indices [N, 2N-1], doubling the number of
queues.

In addition, changing the number of channels was prohibited, as it would
shift the indices.

Remove these two leftovers, as we moved XSK to a new queueing scheme,
starting from index 0.

Fixes: 3db4c85c ("net/mlx5e: xsk: Use queue indices starting from 0 for XSK queues")
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>