Revert "Fix: Amarok crashes on start if a playlist source is unavailable"

This reverts commit 31f75878.

Ralf, this is not the Q_ASSERT in SqlScanResultProcessor.cpp that fails,
it is the metaTrack-> (i.e. KShatedPtr::operator->) that fails!

This needs to be solved differently, reverting this for now.

CCBUG: 313460