Make the scrollbar always thick

Summary:
Second part of D26655, removes the thin scrollbar and makes it always thick
to implement the desired design

BUG: 396747
FIXED-IN: 5.18.0

Test Plan:

Reviewers: Plasma, VDG, Breeze, ngraham, ndavis

Reviewed By: VDG, Breeze, ngraham, ndavis

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T9126

Differential Revision: https://phabricator.kde.org/D26685