btr_cur_pessimistic_update(): Remove extraneous page_zip_write_rec() call.

btr_cur_set_ownership_of_extern_field(): Simplify the logic.

row_upd_rec_in_place(): Make use of parameter "index" in debug assertions.

page_zip_write_rec(): Remove TODO comment about redo log record.
The write will already be covered by higher-level log entries.