Prefer giving KNotification icons rather than fixed-size pixmaps

Summary:
This lets KNotification pick and appropriate size for however it wants
to display the icon.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Maniphest Tasks: T11637

Differential Revision: https://phabricator.kde.org/D24796