Revert "aliases: add nixos-rebuild"

This reverts commit 0b124c1e. 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.)