[kcmkwin/kwindesktop] Remove pointless layout container

Summary:
ColumnLayout only contains one item so is rather pointless, but more
importantly prevents size hints from the formLayout propogating up to
the ScrollViewKCM.

Test Plan: Opened KCM

Reviewers: #kwin, broulik

Reviewed By: broulik

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18297