DAL: remove Dal_slots_statuses from DAL node

This structure is not used anymore if MR !15640 (merged) is merged

  let* confirmed_in_block_hash =
      store_entry_from_published_level
        ~dal_attestation_lag:dal_constants.attestation_lag
        ~published_level
        node_ctxt
    in
    let* processed =
      Node_context.find_slot_status node_ctxt ~confirmed_in_block_hash index
    in

Also see if the hash behind the code below is used elsewhere or if the structure could be removed:

let store_entry_from_published_level ~dal_attestation_lag ~published_level
    node_ctxt =
  Node_context.hash_of_level node_ctxt
  @@ Int32.(
       add (of_int dal_attestation_lag) (Raw_level.to_int32 published_level))