net/mlx5: Remove unused ctx variables

Remove mlx5_priv.ctx_list and ctx_lock which are no longer used after
commit 601c10c8 ("net/mlx5: Delete custom device management logic").

Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>