Why do you want/need to clean this?
We are talking about dead processes and not zombie processes. A dead process in this case is a process that ended and isn't running any more (a zombie process might not be running any more, but is still present and taking resources).
This info is for who's -d option:
The exit field appears for dead processes and contains the termination and exit values (as returned by wait), of the dead process. This can be useful in determining why a process terminated.
The info shown is only present in the wtmp log and does not clutter your machine in any way.
If you do want to get rid of these then rotate the /var/log/wtmp file (which will also remove all other info that might be present).
EDIT: Linux and Unix who command
(a bit more elaborate then the standard manual page)