Even on plasma we shouldn't be using plasma icons

Summary:
We want to use the icons for applications rather than for the shell. It should
work well with the applications color theme rather than the shell's.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: davidedmundson, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D6143