[libcommhistory] Fix infinite recursion in CallModel::deleteEvent

When not in tree mode, the base class implementation of deleteEvent
would call the virtual Event& overload in CallModel, which would end up
calling back into the same base implementation. Fix this by properly
calling the base Event& method from the CallModel overload.
Merge pull request #86 from special/callmodel-delete

[libcommhistory] Fix infinite recursion in CallModel::deleteEvent