Remove references to hydra-postgresql.sql

As of https://github.com/NixOS/hydra/pull/737 (removal of sqlite dependency), the only supported database is Postgresql.

This change removes all references to hydra-postgresql.sql file. This file is generated using a cpp on hydra.sql, but doesn't differ from hydra.sql at all.

Created by  Nikola Knezevic  on June 5, 2020
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