Cleanup EventTag Collection code - remove Fragment concept

Originally EventTag Collections had Collection Fragments (subcollections) concept in the CollectionBase API but it was never implemented in any of the actual Collections technology (neither Relational nor Root). As there is no chance we will ever implement it now, I am removing it completely. 2000 lines of code less makes the Collections a bit lighter and will make the introduction of RNTupleCollections a bit cleaner.

Yet another code cleanup before implementing ATEAM-933