UI: Remove qPixmapFromMimeSource pixmap function

This function dates back to Qt3, and was removed in Qt4. Modern Qt no
longer uses this ui file entry, and in some circumstances the uic
complains about its presence.

src/Mod/Import/Resources/ui/preferences-import.ui

@@ -118,7 +118,6 @@
  </layout>
  </widget>
  <layoutdefault spacing="6" margin="11"/>
- <pixmapfunction>qPixmapFromMimeSource</pixmapfunction>
  <customwidgets>
   <customwidget>
    <class>Gui::PrefRadioButton</class>