You shouldn't need to do that.
The leases file is an internal database of dhcpd. There is no reason for you to look at it.
If you want to know which host is at which IP address, then look at dynamic DNS; that's what it's for.
If you want to know which IP addresses are active, then... also look at dynamic DNS, and try using `dig axfr
If you want to clean it up, then don't -- dhcpd will do that all by itself from time to time.
If you want to do something else, then clarify what exactly it is you want to do. I'm sure there is a better solution to do that which does not require separate lease files.