[plt-scheme] Use of cache.ss by PLaneT is inconsistent

The PLaneT submission instructions suggest testing out a .plt file by
adding it to the planet cache, using it, then removing it.  In my case
this gave me errors when I tried to remove it.  I had just updated my
subversion sources to PLT Scheme 301.12 and my planet cache was empty
(specifically, no cache.ss file).  Using planet --remove complained
that this file did not exist until I specifically required a package
that had to be downloaded remotely.  Either the cache.ss file should
be created when local files are added, or it should not be needed when
packages obtained from local files are deleted.

--
Carl Eastlund
"Cynical, but technically correct."