default to no arrows

Summary:
as the last part of the new design, try to not have arrows
in the scrollbars by default (just change of default,
all the functionality is there)

Test Plan: default is correctly picked up

Reviewers: #plasma, #vdg, hpereiradacosta

Reviewed By: hpereiradacosta

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3378