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/servers/web-apps/codimd/node.nix

@@ -2,7 +2,7 @@
 
 {pkgs ? import <nixpkgs> {
     inherit system;
-  }, system ? builtins.currentSystem, nodejs ? pkgs."nodejs-6_x"}:
+  }, system ? builtins.currentSystem, nodejs ? pkgs."nodejs-8_x"}:
 
 let
   nodeEnv = import ../../../development/node-packages/node-env.nix {
@@ -14,4 +14,4 @@ in
 import ./node-packages.nix {
   inherit (pkgs) fetchurl fetchgit;
   inherit nodeEnv;
-}
+}