Since r1905, innobase_rec_to_mysql() does not require a clustered index record.

row_merge_dup_t: Remove old_table.

row_merge_dup_report(): Do not fetch the clustered index record.  Simply
convert the tuple by innobase_rec_to_mysql().

row_merge_blocks(), row_merge(), row_merge_sort(): Add a TABLE* parameter
for reporting duplicate key values during file sort.

row_merge_read_clustered_index(): Replace UNIV_PAGE_SIZE with the more
appropriate sizeof(mrec_buf_t).