net/mlx5e: Remove unused mlx5e_xsk_first_unused_channel

mlx5e_xsk_first_unused_channel is a leftover from old versions of the
first XSK commit, and it was never used. Remove it.

Fixes: db05815b ("net/mlx5e: Add XSK zero-copy support")
Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>