[SystemTray] Remove scrollbar paddings

Summary: Removes additional paddings. Not needed when scrollbar is no longer overlaying content. See: D26530

Test Plan:
1. Open configuration window
2. No paddings

Reviewers: #plasma_workspaces, #plasma, davidedmundson, broulik, ngraham, nicolasfella

Reviewed By: nicolasfella

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26924