Remove Icons from system tray item tooltips

Summary:
It was decided in T12778 to remove icons from tooltips for panel widgets and system tray
items. This patch implements that for system tray items, and removes the supporting code
from the backend.

Test Plan:
Apply patch
Hover the cursor over system tray items, either in the compat applet or the expanded
representation
No more redundant or inconsistent icons visible

Reviewers: #plasma, davidre, broulik, kmaterka

Reviewed By: kmaterka

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T12778

Differential Revision: https://phabricator.kde.org/D28631