Fix for UNUserNotificationCenter.removeDeliveredNotifications not actually removing notifications

UNUserNotificationCenter+Async.swift

extension UNUserNotificationCenter {
    func removeDeliveredNotificationsAsync(withIdentifiers identifiers: [String]) async {
        // Calls to `UNUserNotificationCenter.current().removeDeliveredNotifications` schedule work
        // on a background thread, with no completion handler or async/await interface. There is no
        // way to know whether the work completed or not. If the notification service extension exits
        // before the work has been completed, the work is cancelled and notifications are not removed.
        //
        // This is really poor API design on Apple's part, and even poorer documentation
        //
        // https://stackoverflow.com/questions/54565979#comment104110958_54680225
        removeDeliveredNotifications(withIdentifiers: identifiers)
        
        // It appears that deliveredNotifications waits for the background work to complete before
        // returning itself (tested on iOS 16). If this proves to be unreliable, then a timer (sigh)
        // or iteration over deliveredNotifications will also work.
        await deliveredNotifications()
    }
}