kernels: 'read-message' no longer calls a non-existent procedure.

* jupyter/kernels.scm (read-message): Remove call to non-existent
'zmq-get-msg-bytevector'.

jupyter/kernels.scm

@@ -347,13 +347,13 @@ milliseconds.  If TIMEOUT expires before a message has been received, return
       (()
        #f)
       ((item rest ...)
-       (let ((socket (poll-item-socket item)))
+       (let* ((socket (poll-item-socket item))
+              (parts  (zmq-get-msg-parts-bytevector socket)))
          ;; Heartbeat messages are raw "bytestrings" that should be echoed
          ;; back right away.
          (if (eq? socket (kernel-heartbeat kernel))
-             (zmq-get-msg-bytevector socket)
-             (parts->message
-              (zmq-get-msg-parts-bytevector socket))))))))
+             parts
+             (parts->message parts)))))))
 
 (define* (send-message kernel message
                        #:key