disable the new class/c implementation for now

the new implementation doesn't provide eq? guarantees
as often as the old one does, so hold off on this until
we can sort that out. Thanks to drdr for pointing out
the problem:

http://drdr.racket-lang.org/28156/pkgs/drracket-pkgs/drracket-test/tests/drracket/teaching-lang-save-file.rkt