Revert "Making nixos keep a ref to its stdenv"

This reverts commit d170c98d13.

niksnut argues that we need smaller system closures, not bigger.

So users facing the trouble of getting gcc rebuilds after nix-collect-garbage
for any minimal nixos configuration change should use other means of
not losing the stdenv output.

One way is to keep one somewhere: nix-build -A stdenv -o stdenv '<nixpkgs>'.
Another may be to use nix.conf options like gc-keep-outputs, gc-keep-derivations
or env-keep-derivations.

nixos/modules/system/activation/top-level.nix

@@ -64,7 +64,6 @@ let
       ln -s ${config.system.build.etc}/etc $out/etc
       ln -s ${config.system.path} $out/sw
       ln -s "$systemd" $out/systemd
-      ln -s ${pkgs.stdenv} $out/stdenv
 
       echo -n "$configurationName" > $out/configuration-name
       echo -n "systemd ${toString config.systemd.package.interfaceVersion}" > $out/init-interface-version