mlx5: remove unused MLX5_DEBUG param in Kconfig

This patch proposes to remove the MLX5_DEBUG kernel configuration
parameter defined in drivers/net/ethernet/mellanox/mlx5/core/Kconfig,
but used nowhere in the makefiles and source code.

This could also be fixed by using this parameter,
but this may be a leftover from driver development...

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>