nix: remove libpq overlay

We have fixed the static build of PostgreSQL upstream, so we don't need
our separate overlay anymore. One more step towards building a static
executable on other platforms.

nix/overlays/default.nix

@@ -3,6 +3,5 @@
   checked-shell-script = import ./checked-shell-script;
   gitignore = import ./gitignore.nix;
   haskell-packages = import ./haskell-packages.nix;
-  postgresql-libpq = import ./postgresql-libpq.nix;
   slocat = import ./slocat.nix;
 }