As part of commit 3c09f148
trx_undo_commit_cleanup() was always invoked with noredo=true.

The impact of this should be that some undo log pages may not be
correctly freed if the server is killed and crash recovery will be
performed. Similarly, if mariabackup --backup is being executed
concurrently with user transaction commits, it could happen that some
undo log pages in the backup will never be marked as free for reuse.

It seems that this bug should not have any user-visible
impact other than some undo pages being wasted.