iputils: drop dontGzipMan, as it's not needed anymore

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

···
 
   buildFlags = "man all ninfod";
 
-  # Stdenv doesn't handle symlinks well for that
-  dontGzipMan = true;
-
   installPhase =
     ''
       mkdir -p $out/sbin $out/bin