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

{F7797241}

Reviewers: #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Maniphest Tasks: T10611

Differential Revision: https://phabricator.kde.org/D25711