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