Remove double scrollbar in appearance settings

Summary:
This patch removes the double scrollbar in the setting window of Yakuake.
The unecessary scrollbar make difficult to find how a new skin can be installed.

Before:

After:

Reviewers: hein, Yakuake

Reviewed By: hein

Subscribers: Yakuake

Tags: Yakuake

Differential Revision: https://phabricator.kde.org/D19833