sources.nix: cleaning

- Remove fetchTarball which is dead code
- Add a header comment specifying sources.nix is a generated file

Main.hs

@@ -608,6 +608,8 @@ pathNixSourcesNix = "nix" </> "sources.nix"
 -- | Glue code between nix and sources.json
 initNixSourcesNixContent :: String
 initNixSourcesNixContent = [s|
+# This file has been generated by Niv.
+
 # A record, from name to path, of the third-party packages
 with rec
 {
@@ -615,13 +617,6 @@ with rec
 
   sources = builtins.fromJSON (builtins.readFile ./sources.json);
 
-  # fetchTarball version that is compatible between all the sources of Nix
-  fetchTarball =
-    { url, sha256 }:
-      if builtins.lessThan builtins.nixVersion "1.12" then
-        builtins.fetchTarball { inherit url; }
-      else
-        builtins.fetchTarball { inherit url sha256; };
   mapAttrs = builtins.mapAttrs or
     (f: set: with builtins;
       listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set)));