Removed special prefix that was being added when naming RNTuples in APR

At the start RNTuple used the same names as corresponging TTrees, with "RNT:" prefix added to not confuse the 2. Now this prefix is removed. Also, RNTuples will now use different names as defined in POOLCore/APRDefaults - except CollectionTree, which is mentioned in 115 places in the code. Maybe something to address later. Athena is now also able to read EventTags in RNTuple format (the previous implementation was not really doing it)