Deleting FirstStageBaseEventClass.

The class was merged with DetailedLogWriterFirstStage. That last one is
the only implementation of FirstStageBaseEventClass : it doesn't need
such a level of inheritance.
Deleting SecondStageBaseEventClass.

The class was merged with DetailedLogWriterSecondStage. That last one is
the only implementation of SecondStageBaseEventClass: it doesn't need
such a level of inheritance.