Fully port away from KIconThemes

Summary:
Depends on D26350

Mostly just removing unused includes
In two places replace KIconLoade::SizeSmall with QStyle equivalent

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: apol

Maniphest Tasks: T11637

Differential Revision: https://phabricator.kde.org/D26352