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.
Migrated from D7686 (view on Phabricator)