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