Drop support for postgres 9.4

nix/overlays/default.nix

@@ -3,5 +3,4 @@
   ghr = import ./ghr;
   haskell-packages = import ./haskell-packages.nix;
   postgresql-default = import ./postgresql-default.nix;
-  postgresql-legacy = import ./postgresql-legacy.nix;
 }

nix/overlays/postgresql-legacy.nix

@@ -1,20 +0,0 @@
-self: super:
-# Overlay that adds legacy versions of PostgreSQL that are supported by
-# PostgREST.
-{
-  # PostgreSQL 9.4 was removed from Nixpkgs with
-  # https://github.com/NixOS/nixpkgs/commit/8e2fc57a80d761c46702c3250e61c1bffe021e25
-  # We pin its parent commit 3b5b9a7 to get the last version that was available.
-  postgresql_9_4 =
-    let
-      rev = "3b5b9a73f59ff93d50156e250203410bdd07f4e0";
-      tarballHash = "0kr25xqbv1ldp72jbmml21pc5hl7xcfqhclv5qxa5f860jddjznk";
-
-      pinnedPkgs =
-        builtins.fetchTarball {
-          url = "https://github.com/nixos/nixpkgs/archive/${rev}.tar.gz";
-          sha256 = tarballHash;
-        };
-    in
-    (import pinnedPkgs { }).pkgs.postgresql_9_4;
-}