leksah: Point to upstream repository for using it

The build in nixpkgs was broken for forever. Trying to fix it would be
painful, as leksah uses a lot of overrides and custom versions. In
addition, leksah works best with the GHC version matching your project,
which means we'd have to make sure it builds for each of our GHC
versions.

pkgs/development/tools/haskell/leksah/default.nix

@@ -1,21 +0,0 @@
-{ stdenv, ghcWithPackages, gtk3, makeWrapper }:
-
-let
-leksahEnv = ghcWithPackages (self: [ self.leksah-server self.leksah self.cabal-install ]);
-in stdenv.mkDerivation {
-  name = "leksah-${leksahEnv.version}";
-
-  buildInputs = [ gtk3 ];
-  nativeBuildInputs = [ makeWrapper ];
-
-  buildCommand = ''
-    mkdir -p $out/bin
-    makeWrapper ${leksahEnv}/bin/leksah $out/bin/leksah \
-      --prefix PATH : "${leksahEnv}/bin" \
-      --prefix XDG_DATA_DIRS : "$GSETTINGS_SCHEMAS_PATH"
-  '';
-
-  meta = {
-    broken = true; # 2018-09-13, no successful hydra build since 2017-08-19
-  };
-}

pkgs/top-level/all-packages.nix

@@ -12726,9 +12726,8 @@ in
 
   lambda-mod-zsh-theme = callPackage ../shells/zsh/lambda-mod-zsh-theme { };
 
-  leksah = callPackage ../development/tools/haskell/leksah {
-    inherit (haskellPackages) ghcWithPackages;
-  };
+  leksah = throw ("To use leksah, refer to the instructions in " +
+    "https://github.com/leksah/leksah.");
 
   libgme = callPackage ../development/libraries/audio/libgme { };