Most Helpful Guy
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.0