[konqpixmapprovider] Don't inherit from KPixmapProvider
Summary: It only defines the pixmapFor method, which we implement ourselves anyway. It is the last non-obsolete usage of KPixmapProvider, so removing it here will allow us to get rid of it entirely eventually.
Test Plan: builds. Basic browsing works, no obvious icons missing
Reviewers: Konqueror, dfaure
Reviewed By: Konqueror, dfaure
Maniphest Tasks: T12310
Differential Revision: https://phabricator.kde.org/D25818