Remove call to no longer needed KIconTheme::assignIconsToContextMenu

Summary:
Qt does set icons itself nowadays, so assignIconToContextMenu got
deprecated.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T11865

Differential Revision: https://phabricator.kde.org/D24960