determined from flag == BTR_CUR_ABORTED. This flag value was previously
never tested for. It was only assigned to, in the only place where
leaf_in_buf_pool would be set FALSE.
btr_cur_search_to_nth_level(): Do not initialize cursor->leaf_in_buf_pool.
btr_pcur_was_buffered(): Remove. The only caller, row_search_index_entry(),
will switch (cursor->flag) and handle BTR_CUR_ABORTED as well.
row_purge_remove_sec_if_poss_low(): Remove the assertions on leaf_in_buf_pool.
