net/mlx5: DR, Remove unneeded functions deceleration

Remove dummy functions declaration, the dummy functions are not needed
since fs_dr is the only one to call mlx5dr and both fs_dr and dr files
depend on the same config flag (MLX5_SW_STEERING).

Fixes: 70605ea5 ("net/mlx5: DR, Expose APIs for direct rule managing")
Signed-off-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>