HPCC-12235 Remove legacy redundant derived RowAggregator class

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #6437 from jakesmith/hpcc-12235

HPCC-12235 Remove legacy redundant derived RowAggregator class

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>