[applet/taskmanager] Remove unnecessary icon size values set on PC3 toolbuttons

Summary:
units.iconSizes.smallMedium is the default size, so setting the
icon size to this has no effect

Test Plan:
No change whatsoever in the task manager media playback tooltip
icon sizes

Reviewers: VDG, Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28236