linux-headers: Remove stray linux headers 3.18 nix file

In dced724c this derivation was
unexposed along (in all-packages.nix) with the removal of linux 3.18[1].
I think this file was left behind by mistake.

[1]: https://github.com/NixOS/nixpkgs/commit/dced724c009a1646475373cc597ada385d46bde6#diff-036410e9211b4336186fc613f7200b12L11174