devlink: Remove unused functions devlink_rate_leaf_create/destroy

The previous patch removed the last usage of the functions
devlink_rate_leaf_create() and devlink_rate_nodes_destroy(). Thus,
remove these 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>