[racket] McFly discontinuing PLaneT support

If you're using McFly, be advised that subsequent releases will no 
longer support PLaneT.

(Reason: I need to rework McFly for the new package system, but McFly is 
very PLaneT-centric, and preserving the PLaneT support would be an 
impractical amount of extra work.  I'm going to throw out almost all of 
the code.  The good news is that, using the new package system with the 
new McFly, you probably won't have to change your `info.rkt` at all -- 
you can even keep your `mcfly-planet` variable (if no `mcfly-version` is 
defined).  And, if you let McFly maintain a Web hosting (`rsync` 
staging?) filesystem tree for you, that will support PLaneT-like 
"version X.Y or later Y" version specs in `require` forms, and more.  
Although you still won't be able to use multiple versions of the same 
package at the same time in a Racket setup right now.)

Neil