KUrlPixmapProvider is a weird little thing. The only thing it does is getting a pixmap for a given URL. It therefore uses KIO::pixmapForUrl, which uses KIO::iconNameForUrl and then loads a pixmap by this name.
The only place where this is actually used is KDirSelectDialog to feed it into KHistoryComboBox to give the history nice little icons.
This is also the only place where setPixmapProvider() from KHistoryCombobox is actually used.
Given that the icons are loaded by name anyway and QCombobox takes a QIcon (and thus QIcon::fromTheme) we should find a way to feed the icon name directly into the combobox without the pixmap madness in between.
KUrlPixmapProvider and KonqPixmapProvider are also the only users of KPixmapProvider, which then can be killed as well