Remove remaining icon from applet with custom tooltip

Summary:
Last bits needed for T12778.

CCBUG: 420934

Test Plan: {F8295118}

Reviewers: bugseforuns, #plasma, #vdg, ndavis

Reviewed By: #vdg, ndavis

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T12778

Differential Revision: https://phabricator.kde.org/D29516