Delete OCS ID from DB if none is specified

If no OCS ID is specified in appinfo.xml and an app update is triggered and a OCS ID is stored in the DB we should clean the value.

Ref https://github.com/owncloud/activity/issues/320#issuecomment-117937748

lib/private/app.php

@@ -1174,6 +1174,8 @@ class OC_App {
 		$appData = self::getAppInfo($appId);
 		if (array_key_exists('ocsid', $appData)) {
 			OC_Appconfig::setValue($appId, 'ocsid', $appData['ocsid']);
+		} elseif(OC_Appconfig::getValue($appId, 'ocsid', null) !== null) {
+			OC_Appconfig::deleteKey($appId, 'ocsid');
 		}
 		foreach ($appData['remote'] as $name => $path) {
 			OCP\CONFIG::setAppValue('core', 'remote_' . $name, $appId . '/' . $path);