rdma-core: remove unused build inputs

ethtool and iproute2 are listed in the "recommends" section
of upstream's [packaging docs]. they aren't actually used by
rdma-core, and don't appear in its runtime closure. it's better
read as "if you find rdma-core useful, you should also check out
{iproute2,ethtool,...}".

[packaging docs]: https://github.com/linux-rdma/rdma-core/blob/f5442854eccd4cc9d1f20bf200171c26e6859381/debian/control#L32

pkgs/by-name/rd/rdma-core/package.nix

@@ -6,8 +6,6 @@
   pkg-config,
   docutils,
   pandoc,
-  ethtool,
-  iproute2,
   libnl,
   udev,
   udevCheckHook,
@@ -44,8 +42,6 @@ stdenv.mkDerivation (finalAttrs: {
   ];
 
   buildInputs = [
-    ethtool
-    iproute2
     libnl
     perl
     udev