Don't create pixmaps for logmode icons

Summary: The icons are specified by name and deal with QIcon all the way. No need to create pixmaps.

Test Plan: Logmode selection in config UI still has icons

Reviewers: #plasma, vmatyushin, rthomsen, davidedmundson

Reviewed By: #plasma, davidedmundson

Maniphest Tasks: T11637

Differential Revision: https://phabricator.kde.org/D26345