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.

src/sql/update-dbix-harness.sh

@@ -33,7 +33,7 @@ pg_ctl -D "$data" \
 
 createdb -h "$socket" "$dbname"
 
-psql -h "$socket" "$dbname" -f ./hydra-postgresql.sql
+psql -h "$socket" "$dbname" -f ./hydra.sql
 
 perl -I ../lib \
      -MDBIx::Class::Schema::Loader=make_schema_at,dump_to_dir:../lib \