[R6RS] corrected define-syntax for case-lambda

[R. Kent Dybvig]

I have replaced the buggy definition of case-lambda in libderived.tex with
a correct and shorter one (still written using syntax-rules) in
misclib.tex.  (I believe it was in libderived.tex only because it seemed
too long and obtuse for misclib.tex, but I feel that is no longer the
case.)

As it was the only definition given in libderived.tex, that file and
section is now obsolete and should be eliminated.  I haven't done so.

Kent