[python] builder: dedup organisms table

This is a minimal diff to drop the duplicate rows from the `census["census_info"]["organisms"]` table arising from the builder's distinct spatial & non-spatial ExperimentSpecs for human and mouse. Rather than the simple deduplication, perhaps it'd be useful to add a `modality` or `[modalities]` column or something like that, but that would be a schema change; we can discuss that or perhaps just leave behind a ticket.