[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