Remove "Mark all as seen" button from the Notifications dropdown

[#1]

The "Mark all as seen" button that appears in the notification dropdown has been very frustrating for some of our users. It only appears when you put the mouse arrow over it and when our users are moving fast it is often accidentally clicked and cannot be undone.

I think the button that is on the full notification page after clicking on "Notifications" is good enough for people who want to utilize this.