Don't set Breeze as icon theme

Summary: Remove code, so it can use the system icon theme.

Test Plan: ISOImageWriter now uses the system icon theme.

Reviewers: VDG, jriddell, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, jriddell, VDG

Differential Revision: https://phabricator.kde.org/D20650