Remove nodejs-6_x which is about to enter EOL

- Remove nodejs-6_x
- Set nodejs / nodejs-slim to nodejs-8_x / nodejs-slim-8_x
- Re-generate node2nix generated files using nodejs-8_x instead

pkgs/misc/base16-builder/default.nix

@@ -1,14 +1,6 @@
 { stdenv, pkgs }:
 
 let
-  # node-packages*.nix generated via:
-  #
-  # % node2nix --input node-packages.json \
-  #            --output node-packages-generated.nix \
-  #            --composition node-packages.nix \
-  #            --node-env ./../../development/node-packages/node-env.nix \
-  #            --pkg-name nodejs-6_x
-  #
   nodePackages = import ./node-packages.nix {
     inherit pkgs;
     inherit (stdenv.hostPlatform) system;