Revert "aliases: add nixos-rebuild"

This reverts commit 0b124c1e9152959cd9de9de46c4a4d9146a818e6. We
should really stop adding things that are not packages to
all-packages.nix. For example, having nixos-rebuild.nix in
all-packages.nix causes 'nix-env -qa' to evaluate a NixOS
configuration, which obviously is not good for performance. (We should
probably also remove the 'nixos' attribute from all-packages.nix, but
at least that's a function so nix-env will ignore it.)

pkgs/top-level/aliases.nix

···
     (import self.path { localSystem = { inherit system; }; });
   callPackage_i686 = pkgsi686Linux.callPackage;
 
-  inherit (nixos {}) nixos-rebuild;
-
   inherit (ocaml-ng) # added 2016-09-14
     ocamlPackages_3_10_0 ocamlPackages_3_11_2 ocamlPackages_3_12_1
     ocamlPackages_4_00_1 ocamlPackages_4_01_0 ocamlPackages_4_02