[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