[plt-scheme] planet temporary files

I just noticed that there were temporary planet files in /var/tmp. I 
blew them all away but was wondering if they were really necessary to 
begin with and should planet erase them after the installation completes.

$ ls /var/tmp
-rw-r--r--  1 kazzmir 5.7M Nov 17 21:36 planettmp11638174081163817408942.plt
-rw-r--r--  1 kazzmir 5.7M Oct 30 23:19 planettmp11622683551162268355193.plt
-rw-r--r--  1 kazzmir 5.7M Dec 10 11:07 planettmp11657668501165766850823.plt