update Notifications.clearAll() method to really clear all

Author: nan-li

iOS_SDK/OneSignalSDK/OneSignalNotifications/OSNotificationsManager.m

@@ -337,6 +337,8 @@ + (void)presentAppSettings {
 }
 
 + (void)clearAll {
+    [[UNUserNotificationCenter currentNotificationCenter] removeAllDeliveredNotifications];
+    // TODO: Determine if we also need to call clearBadgeCount
     [self clearBadgeCount:false];
 }