In forms.inc.php:1210 in the getItemFormData() function there's a block of code, that returns the available host groups. They don't seem to be used anywhere after.

// host groups
$data['hostGroups'] = API::HostGroup()->get(array(
'editable' => true,
'output' => API_OUTPUT_EXTEND
));
order_result($data['hostGroups'], 'name');

This code should be removed.