CRM-12497 API 'getfields' method returns all custom fields

'getfields' is returning every single custom field, no matter which API you call it on.

In api/v3/utils.php, line 1300, the culprit line is right there:
$fields += _civicrm_api_get_custom_fields($entity, $params);

What's confusing to me is that github says this line existed when we pulled in from SVN on 2/28/13 - but the behavior isn't in 4.3. I'm not too fussed about that; it's just that I don't know who to ask about why it's there!