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/script/hydra-init

@@ -25,7 +25,7 @@ my @tables = $dbh->tables;
 if (! grep { /SchemaVersion/i } @tables) {
     print STDERR "initialising the Hydra database schema...\n";
     my $schema = read_file(
-        $dbh->{Driver}->{Name} eq 'Pg' ? "$home/sql/hydra-postgresql.sql" :
+        $dbh->{Driver}->{Name} eq 'Pg' ? "$home/sql/hydra.sql" :
         die "unsupported database type $dbh->{Driver}->{Name}\n");
     my @statements = $sql_splitter->split($schema);
     eval {