nixos-checkout: Remove

This command was useful when NixOS was spread across multiple
repositories, but now it's pretty pointless (and obfuscates what
happens, i.e. "git clone git://github.com/NixOS/nixpkgs.git").

nixos/modules/module-list.nix

@@ -47,7 +47,6 @@
   ./i18n/input-method/nabi.nix
   ./i18n/input-method/uim.nix
   ./installer/tools/auto-upgrade.nix
-  ./installer/tools/nixos-checkout.nix
   ./installer/tools/tools.nix
   ./misc/assertions.nix
   ./misc/crashdump.nix