Allow to disable deletion tracking.

This is responsible for big amounts of memory consumption.

Here it increases in 50MB after each restart of akonadi or
ETMCalendar reset (which call the calendar delete operation internally).

Memory usage can also be seen without restart akonadi, just by
deleting lots of events.

I suggest deprecating deletedEvent(), deletedTodo() and deletedJournal()
which is weird API that we don't use anywhere in KDE.