kernel: Don't patchELF manually

AFAICT this is done by stdenv nowadays:

https://github.com/NixOS/nixpkgs/blob/bde82098b88861a27faadc8a69b597510e56d846/pkgs/development/tools/misc/patchelf/setup-hook.sh#L5
https://github.com/NixOS/nixpkgs/blob/bde82098b88861a27faadc8a69b597510e56d846/pkgs/stdenv/generic/setup.sh#L737

Might help /be somehow related to these mysterious errors that seem to
occur sometimes (?):

http://hydra.nixos.org/build/34131589/nixlog/1/raw

pkgs/os-specific/linux/kernel/manual-config.nix

@@ -190,15 +190,6 @@ let
           $installFlags "''${installFlagsArray[@]}"
       '');
 
-      # !!! This leaves references to gcc in $dev
-      # that we might be able to avoid
-      postFixup = if isModular then ''
-        # !!! Should this be part of stdenv? Also patchELF should take an argument...
-        prefix=$dev
-        patchELF
-        prefix=$out
-      '' else null;
-
       meta = {
         description =
           "The Linux kernel" +