net/mlx5: Remove unused MLX5_*_DOORBELL_LOCK macros

MLX5_*_DOORBELL_LOCK macros provided a way to avoid locking for
mlx5_write64 on 64-bit platforms where it's not necessary. Currently all
calls to mlx5_write64 don't use a spinlock, so the macros became unused.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>