devlink: Remove unused function devlink_rate_nodes_destroy

The previous patch removed the last usage of the function
devlink_rate_nodes_destroy(). Thus, remove this function from devlink
API.

Signed-off-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>