Cleanup EventSelectorAthenaPool properties

This MR mainly cleans up EventSelectorAthenaPool properties. Only the one instance of ToolHandleArray could not be modernised as some upstream Gaudi changes are needed.

All unused properties have been removed so we should run this over all unit tests (the base AthenaPoolTest tests run fine locally).

This is a preparation to use this class as a base class of DoubleEventSelectorAthenaPool.

@gemmeren, it would be nice if you can have a quick look. (FYI @jchapman, @mmuskinj)