Remove double margin in KCM UI
Summary:
As per http://agateau.com/2007/setmargin0/ and apparently still relevant
10 years later.
Test Plan:
Looked at it.
Now the QTabWidget lines up with the buttons on the bottom
Reviewers: Plasma, apol
Reviewed By: apol
Subscribers: apol, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D9769