Don't covert icons to pixmaps multiple times

Summary: We don't need to convert the icon which is loaded by name to a pixmap here. The conversion happens later in the DecorationRole data() call.

Test Plan:
Open archive. Icons in the file list appear and are properly sized.
Trigger overwrite dialog. Icons appear and are properly sized.

Verified performance improvement with hotspot (see highlighted entries)

before:
{F7770370}

after:
{F7770371}

Reviewers: #ark, elvisangelaccio

Reviewed By: #ark, elvisangelaccio

Subscribers: elvisangelaccio, kde-utils-devel

Tags: #ark

Differential Revision: https://phabricator.kde.org/D25377