net/mlx5: Remove pointless devlink_rate checks

It is guaranteed that the devlink rate leaf is created during init paths.
No need to check during cleanup. Remove the checks.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>