Since we can't load Qt4 and Qt5 based libraries in one process
prevent loading the conflicting config backends based on the VCL
backend.

Also removes the reference to the long gone KDE3 backend.

Change-Id: I7a9bc449ebf825b47882f8bd207496a2a9597608
Reviewed-on: https://gerrit.libreoffice.org/60764
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>