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/Makefile.am

@@ -1,13 +1,9 @@
 sqldir = $(libexecdir)/hydra/sql
 nobase_dist_sql_DATA =				\
-  hydra-postgresql.sql				\
   hydra.sql					\
   test.sql					\
   upgrade-*.sql					\
   update-dbix.pl
 
-hydra-postgresql.sql: hydra.sql
-	cpp -P -E -traditional-cpp -DPOSTGRESQL hydra.sql > $@ || rm -f $@
-
-update-dbix: hydra-postgresql.sql
+update-dbix: hydra.sql
 	./update-dbix-harness.sh