Remove group index row counters

They were only ever used during statistics computation, which was already inaccurate when multi-transaction configuration was enabled. Use the row count of the leaf most table for the input estimate (as the optimizer does).

Also allowed AbstractStore#clear() to stop returning whether or not something was removed.

Minor tweak to AbstoreStore#writeRowInternal() to not bump row count until *all* (unique) indexes have been processed, for the same reason as the existing comment.

Remove IndexEntryCountIT entirely as the API it used was gone and just a surrogate for the table row count anyway.