iproute: remove unused patch

In f18efaf26e, the "patches" attribute was erroneously renamed to
"patch". If you follow the original bug report[1], you'll see that this
has long since been fixed upstream (using a different patch).

1: https://bugs.gentoo.org/show_bug.cgi?id=331447

pkgs/os-specific/linux/iproute/default.nix

@@ -8,8 +8,6 @@ stdenv.mkDerivation rec {
     sha256 = "0vz6m2k6hdrjlg4x0r3cd75lg9ysmndbsp35pm8494zvksc7l1vk";
   };
 
-  patch = [ ./vpnc.patch ];
-
   preConfigure = ''
     patchShebangs ./configure
     sed -e '/ARPDDIR/d' -i Makefile