docs: remove PostgreSQL local setup

We no longer support exporting the dataset as PostgreSQL dumps. It's
pretty much useless for big data analysis; we rather encourage
researchers to use big data engine (Hadoop, Hive, Presto...) which all
support the ORC format.

Alternatives to import the dataset on PostgreSQL include
https://github.com/HighgoSoftware/orc_fdw/ , or using the swh mirroring
pipeline to spawn a local storage instance with a PostgreSQL backend.