[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