HPCC-12235 Remove legacy redundant derived RowAggregator class

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
stop() can't be called asynchronously, i.e. no rows can be requested from the activity as it is called.

There is a separate reset() phase which is called on execute, only if the graph has been previously started.
In this case, it won't make any practical difference, it will either be called on next execute, or destroyed after last.
Main reason it's there is that stop() tends to always be overloaded, whereas reset() often doesn't need to be.