This removes the negative top margin, as it's already removed by the NotificationPopup

Summary: The negative top margin was not necessary and causes missing round corners.

Test Plan:
No visual change on master
With D29479, rounded corners should appear again

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29490