gnused: Remove crossAttrs

Use a cleaner help2man trick from coreutils

pkgs/tools/text/gnused/default.nix

@@ -14,13 +14,8 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ perl ];
   preConfigure = "patchShebangs ./build-aux/help2man";
 
-  crossAttrs = {
-    # The tarball ships with a fine prebuilt manpage, but the make rules try to rebuild it,
-    # which won't work when cross compiling as help2man needs to execute the binaries.
-    postConfigure = ''
-      sed -i Makefile -e 's|doc/sed\.1:|dummy:|'
-    '';
-  };
+  # Prevents attempts of running 'help2man' on cross-built binaries.
+  PERL = if stdenv.hostPlatform == stdenv.buildPlatform then null else "missing";
 
   meta = {
     homepage = http://www.gnu.org/software/sed/;