Fix crash in QConcatenateTablesProxyModel when plasma shuts down

Fix crash in QConcatenateTablesProxyModel when plasma shuts down

This is a bit speculative, but usually what happens with these crashes is:

During destruction, the source models of the concat model are destroyed. Before the proxy model is also deleted, some codepath triggers a data lookup, which calls into the dangling pointers of the deleted source models. Fix by explicitely removing the source models before deleting them.

A different fix could also be to overhaul the entire ownership management of the models here, but that's more involved...

SENTRY: PLASMA-WORKSPACE-56PR (second most common plasmashell crash at the moment)