DAL: simplify function cells_of_level

Starting from Rio, the function cells_of_level exposed by the DAL plugin can work with just an attestation level as an argument instead of block_info.

This MR proposes to make the simplification. For Quebec, we (re)-fetch the block info locally, as its metadata is needed. This should not be an issue in practice, as Rio has been activated for almost one month (Quebec's cells_of_level function will likely not be called).