Remove backwards compatibility with V1 code

This was a nice idea, but in practice it did not work unless you
were explicitly extracting just `config`, `overlays` and `system`
(see #542).

```
~/iohk/haskell.nix$ nix-build -E 'import <nixpkgs> (import ./.)'
error: anonymous function at /nix/store/f7mwvx7gl8lwjiqmpv60xapc3crvc474-nixpkgs-20.03pre194824.1f56d679f40/nixpkgs/pkgs/top-level/default.nix:20:1 called with unexpected argument '__functor', at /nix/store/f7mwvx7gl8lwjiqmpv60xapc3crvc474-nixpkgs-20.03pre194824.1f56d679f40/nixpkgs/pkgs/top-level/impure.nix:84:1
```

This change will replace #542 errors with #709 errors.

Author: hamishmack

default.nix

@@ -1,49 +1,33 @@
-let haskellNix = {
-      checkMaterialization ? false,  # Allows us to easily switch on materialization checking
-      defaultCompilerNixName ? null, # Quick way to override the default compiler e.g. "ghc883"
-      system ? builtins.currentSystem,
-      sourcesOverride ? {},
-      ... }@args: rec {
-    sources = (import ./nix/sources.nix) // sourcesOverride;
+{ checkMaterialization ? false  # Allows us to easily switch on materialization checking
+, defaultCompilerNixName ? null # Quick way to override the default compiler e.g. "ghc883"
+, system ? builtins.currentSystem
+, sourcesOverride ? {}
+, ... }@args: rec {
+  sources = (import ./nix/sources.nix) // sourcesOverride;
 
-    config   = import ./config.nix;
-    overlays = [ allOverlays.combined ] ++ (
-      if checkMaterialization == true
-        then [(
-          final: prev: {
-            haskell-nix = prev.haskell-nix // {
-              checkMaterialization = true;
-            };
-          }
-        )]
-        else []
-    ) ++ (
-      if defaultCompilerNixName != null
-        then [(
-          final: prev: {
-            haskell-nix = prev.haskell-nix // {
-              inherit defaultCompilerNixName;
-            };
-          }
-        )]
-        else []
-    );
-    allOverlays = import ./overlays args;
-    nixpkgsArgs = { inherit config overlays system; };
-    pkgs = import sources.nixpkgs-default nixpkgsArgs;
-  };
-
-  haskellNixV1 = (haskellNix {}).nixpkgsArgs;
-  haskellNixV2 = haskellNix;
-
-  v1DeprecationMessage = "Version 1 is deprecated: use version 2 (nixpkgs arguments are available as the `nixpkgsArgs` attribute of version 2)";
-# If no arguments, then you get V1
-# I'd like to make importing directly issue a warning, but I couldn't figure out a way to make it happen
-in haskellNixV1 // {
-  __functor = _: { version ? 2, ... }@args:
-    if version == 1
-    then builtins.trace v1DeprecationMessage haskellNixV1
-    else if version == 2
-    then haskellNixV2 args
-    else builtins.throw ("haskell.nix: unknown version: " + (builtins.toString version));
+  config   = import ./config.nix;
+  overlays = [ allOverlays.combined ] ++ (
+    if checkMaterialization == true
+      then [(
+        final: prev: {
+          haskell-nix = prev.haskell-nix // {
+            checkMaterialization = true;
+          };
+        }
+      )]
+      else []
+  ) ++ (
+    if defaultCompilerNixName != null
+      then [(
+        final: prev: {
+          haskell-nix = prev.haskell-nix // {
+            inherit defaultCompilerNixName;
+          };
+        }
+      )]
+      else []
+  );
+  allOverlays = import ./overlays args;
+  nixpkgsArgs = { inherit config overlays system; };
+  pkgs = import sources.nixpkgs-default nixpkgsArgs;
 }