The redo log record MLOG_UNDO_HDR_DISCARD is never written, and
the function trx_undo_discard_latest_update_undo() is never called
outside crash recovery.

Starting with MariaDB 10.2.2, crash recovery from an older InnoDB
version is refused. Therefore, we can safely remove the code for
parsing this long-unused redo log record type.

The use of MLOG_UNDO_HDR_DISCARD was removed by Heikki Tuuri,
the original designer and author of InnoDB, more than 13 years ago:

  commit 3caab0f3
  Author: unknown <heikki@hundin.mysql.fi>
  Date:   Thu Mar 18 14:57:22 2004 +0200

    trx0undo.h, trx0undo.c, trx0trx.c:
      Update an obsolete comment about trx commit: we can no longer call
      trx_undo_update_cleanup_by_discard(), and actually, the idea to
      call it was erroneous, it cannot work