Remove unused iftop dependency

The network throughput implementation uses /proc/net/dev parsing
instead of iftop, so the iftop dependency is not needed.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

nix/package.nix

@@ -2,7 +2,6 @@
   lib,
   beamPackages,
   src,
-  iftop,
 }:
 
 beamPackages.mixRelease rec {
@@ -11,9 +10,6 @@ beamPackages.mixRelease rec {
 
   inherit src;
 
-  # Runtime dependencies
-  buildInputs = [ iftop ];
-
   # Disable distributed Erlang to avoid COOKIE requirement
   postInstall = ''
         # Create wrapper script that sets proper environment including COOKIE