Remove runtime backend switching.

It was decided long ago that it is not worth the effort.
No currently maintained backend actually supports it, so that
Phonon git master actually stops trying to switch altogether.
Through this change the KCM will stop making Phonon switch
at runtime from 4.8 onwards and instead present a nice
KMessageWidget informing the user that they will need to
login again to apply the changes.

This will prevent backend change related crashes (in KDE >=
4.8) neverminding the version of Phonon.

CCBUG: 210427
CCMAIL: kde-multimedia@kde.org