I know I'm so late and you already you got the answer, but I didn't see a good opinion from guys' side so here it is :D
Click on "bell" (on PC) version or 'view more notifications' or just click on this link :- www.girlsaskguys.com/.../notifications to open notification page. Then click the "Mark all as read" button. Problem solved.
PS: it may vanish an important @mention notification which would have been appeared after 48 hours or so. So, do it at your own risk.
If you go to your actual notifications page and click the "Mark all as read" button, that should clear all of your notifications. If you still have the notification after doing that, then just let me know and we will look into it :)