Fix rocksdb_read_free_rpl test to properly count rows corresponding to broken index entries

The rocksdb.rocksdb_read_free_rpl test checks scenarios when secondary key may loose some records or have extra records due to inconsistencies between table on primary and secondary servers when read free replication is used. Currently "select count(column_name) from table_name force index(column_name)" is used for this purpose. In MySQL 8.0 there were changes to count() behavior and now it doesn't take into account rows corresponding to broken index entries. Simple "select column_name from table_name" can be used instead to reveal the issue with broken index entries.