net/mlx5e: Remove redundant active_channels indication

Now, when all channels stats are saved regardless of the channel's state
{open, closed}, we can safely remove this indication and the stats spin
lock which protects it.

Fixes: 76c3810bade3 ("net/mlx5e: Avoid reset netdev stats on configuration changes")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>