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