Don't relay write-special for stdout/stderr; fixes #677

Although we handle write-special in a temporary values port in
print.rkt, for convertible image values, we don't need to generally
for current-output-port or current-error-port.

We were already ignoring non-image special values in the Emacs front
end. So handling write-special in the general ports in the back end
only means we might try, unnecessarily, to elisp-write some arbitrary
Racket value, and sometimes fail. (Whatever the student langs and
DrRacket are doing in this regard with write-special, beyond images,
we can ignore for now, and maybe forever.)

racket/repl-output.rkt

@@ -94,13 +94,8 @@
     (async-channel-put repl-output-channel
                        (repl-output kind (subbytes bstr start end)))
     (- end start))
-  (define (write-out-special v _non-block? _breakable?)
-    (async-channel-put repl-output-channel
-                       (repl-output special-kind v))
-    #t)
   (define close void)
   (make-output-port name
                     repl-output-channel
                     write-out
-                    close
-                    write-out-special))
+                    close))