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/static.nix

@@ -18,23 +18,6 @@ let
         # Cross compiling with native bignum works better than with gmp
         enableNativeBignum = true;
       };
-
-      overrides = pkgs.lib.composeExtensions old.overrides (_: prev: {
-        postgresql-libpq = (lib.overrideCabal prev.postgresql-libpq {
-          # TODO: This section can be simplified when this PR has made it's way to us:
-          #  https://github.com/NixOS/nixpkgs/pull/286370
-          # Additionally, we need to use the default version in nixpkgs, otherwise the
-          # override will not be active as well.
-          # Using use-pkg-config flag, because pg_config won't work when cross-compiling
-          configureFlags = [ "-fuse-pkg-config" ];
-          # postgresql doesn't build in the fully static overlay - but the default
-          # derivation is built with static libraries anyway.
-          libraryPkgconfigDepends = [ pkgsStatic.libpq ];
-          librarySystemDepends = [ ];
-        }).overrideAttrs (_: prevAttrs: {
-          buildInputs = prevAttrs.buildInputs ++ [ pkgsStatic.openssl ];
-        });
-      });
     });
 
   makeExecutableStatic = drv: pkgs.lib.pipe drv [