Revert "Fix looping through attachment ids."

This looping goes over existing data from the internal cache, so the hash
already should have been built.

This reverts commit a939418.

framework/Kolab_Storage/lib/Horde/Kolab/Storage/Cache/Data.php

@@ -458,7 +458,7 @@ public function store(array $objects,
             foreach ($delete as $obid => $object_id) {
                 $object = $this->_data[self::OBJECTS][$object_id];
                 if (isset($object['_attachments'])) {
-                    foreach (array_keys($object['_attachments']) as $id) {
+                    foreach ($object['_attachments']['id'] as $id) {
                         $this->_cache->deleteAttachment(
                             $this->getDataId(), $obid, $id
                         );