Only multiply the icon size if Qt isn't doing it already

Summary:
This multiplication is redundant with Qt and will introduce awkward
scaling to images and poor rendering.

BUG: 390076

Reviewers: #kirigami, mart

Reviewed By: #kirigami, mart

Subscribers: plasma-devel

Tags: #kirigami

Differential Revision: https://phabricator.kde.org/D10413