`$shortcodes_and_callbacks_to_remove` isn't being correctly initialised, so [this line](https://github.com/Automattic/jetpack/blob/689f7ae0fed3efc4374f05692b10969aef633dbb/sync/class.jetpack-sync-module-posts.php#L198) causes a PHP notice.
$shortcodes_and_callbacks_to_removeisn't being correctly initialised, so this line causes a PHP notice.