Allow direct URL calls to REST::ajax if debug is enabled

Currently there's no decent way to debug ajax calls that load into widgets (e.g. contactListQuery for autocomplete) because we've appropriately blocked calls to CRM_Utils_REST:ajax that aren't made by javascript clients.

However, sometimes developers need to debug results of these calls in the browser. I've modifed ajax() to allow direct calls via URL if CiviCRM debug is enabled ($config->debug is true).

Lobo and I also noticed several spots in REST.php where the $config object was being passed around, but never used. I've cleaned up / removed those references / parameters.