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