Simplify __rec_append_orig_value

In __rec_append_orig_value, we append a tombstone to the update chain if there is no cell.

if (unpack == NULL || unpack->type == WT_CELL_DEL)

        WT_RET(__wt_update_alloc(session, NULL, &append, &size, WT_UPDATE_TOMBSTONE));

I think this is no longer required in durable history and we don't need to append anything if there is no cell or the cell is deleted.

Also we don't need to append the original value if the stop time pair is globally visible.