Remove unclear comment from hardware-configuration.nix

It is likely that hardware-configuration.nix is generated once (just like configuration.nix) and never regenerated again. Looking into `nixos-option.sh`:

```
hardware_config="${NIXOS_CONFIG%/configuration.nix}/hardware-configuration.nix"
  if test -e "$hardware_config"; then
    echo "A hardware configuration file exists, generation skipped."
  else
    echo "Generating a hardware configuration file in $hardware_config..."
    nixos-hardware-scan > "$hardware_config"
  fi
```