flake.nix: refactoring

- Remove default redundant default arguments in nix/default.nix
- Remove unused exports in flake

default.nix

@@ -17,8 +17,11 @@
       stackage = sources."stackage.nix";
     };
   }
+  # haskell-nix has to be used differently in flakes/no-flakes scenarios:
+  # - When imported from flakes, 'haskellNix.overlay' needs to be passed here.
+  # - When imported from default.nix without flakes, default to haskellNix.overlays
 , haskellNixOverlays ? haskellNix.overlays
-, packages ? import ./nix { inherit system sources crossSystem config sourcesOverride haskellNixOverlays checkMaterialization enableHaskellProfiling; }
+, packages ? import ./nix { inherit system sources crossSystem config sourcesOverride haskellNix haskellNixOverlays checkMaterialization enableHaskellProfiling; }
   # An explicit git rev to use, passed when we are in Hydra
   # Whether to check that the pinned shas for haskell.nix are correct. We want this to be
   # false, generally, since it does more work, but we set it to true in the CI

flake.nix

@@ -15,9 +15,7 @@
           haskellNixOverlays = [ haskell-nix.overlay ];
         };
 
-        inherit (topLevel) pkgs plutus ownOverlays;
-        inherit (plutus) haskell iohkNix;
-        inherit (plutus.lib) buildPursPackage buildNodeModules filterNpm gitignore-nix;
+        inherit (topLevel) pkgs plutus;
       in
       rec {
         packages = rec {

nix/default.nix

@@ -3,17 +3,9 @@
 , config ? { }
 , overlays ? [ ]
 , sourcesOverride ? { }
-, sources ? import ./sources.nix { } // sourcesOverride
-, haskellNix ? import sources."haskell.nix" {
-    sourcesOverride = {
-      hackage = sources."hackage.nix";
-      stackage = sources."stackage.nix";
-    };
-  }
-  # haskell-nix has to be used differently in flakes/no-flakes scenarios:
-  # - When imported from flakes, 'haskellNix.overlay' needs to be passed here.
-  # - When imported from default.nix without flakes, default to haskellNix.overlays
-, haskellNixOverlays ? haskellNix.overlays
+, sources
+, haskellNix
+, haskellNixOverlays
 , checkMaterialization ? false
 , enableHaskellProfiling ? false
 }:
@@ -38,5 +30,5 @@ let
 
 in
 {
-  inherit pkgs plutus sources ownOverlays;
+  inherit pkgs plutus sources;
 }