Get rid of dependency to SQLite

SQLite isn't properly supported by Hydra for a few years now[1], but
Hydra still depends on it. Apart from a slightly bigger closure this can
cause confusion by users since Hydra picks up SQLite rather than
PostgreSQL by default if HYDRA_DBI isn't configured properly[2]

[1] 78974abb69
[2] https://logs.nix.samueldr.com/nixos-dev/2020-04-10#3297342;

.gitignore

@@ -32,3 +32,4 @@ Makefile.in
 hydra-config.h
 hydra-config.h.in
 result
+tests/jobs/config.nix