TrxUndoRsegs::append(): Remove.

TrxUndoRsegsIterator::set_next(): Add a debug assertion that
demonstrates that the merging of rollback segments never occurs.
Since MDEV-12289 or earlier, MariaDB 10.2 will not make any
temporary undo log accessible to the purge subsystem.
(Also MySQL 5.7 would skip the purge of any undo log for
temporary tables, but not before parsing and buffering those
temporary undo log records.)