Remove scrollbar behavior option in GTK KCM
Summary:
Since this setting now in sync with Plasma and Qt apps one, there is no need in it to be in separate KCM.
Depends on D25703
Test Plan:
Check if nothing breaks
Reviewers: VDG, ngraham
Reviewed By: VDG, ngraham
Subscribers: ngraham, plasma-devel
Tags: Plasma
Maniphest Tasks: T10611
Differential Revision: https://phabricator.kde.org/D25711