Revert "nixos: set nixos in nixPath"

This reverts commit 67c8c49177.

'nix run nixos.firefox' is *not* supposed to work - the Nix 2.x
interface attempts to standardize on nixpkgs.*, to get rid of the
nixos/nixpkgs confusion that existed with the channels interface. So
let's not bring that confusion back.

nixos/modules/services/misc/nix-daemon.nix

@@ -347,7 +347,6 @@ in
           [
             "$HOME/.nix-defexpr/channels"
             "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos"
-            "nixos=/nix/var/nix/profiles/per-user/root/channels/nixos"
             "nixos-config=/etc/nixos/configuration.nix"
             "/nix/var/nix/profiles/per-user/root/channels"
           ];