Fix 5_13

It turns out that there's a built in procedure for removing duplicates from lists.

5_13.scm

@@ -14,15 +14,11 @@
         (else '())))
 (define (not-null? x)
   (not (null? x)))
-(define (remove-duplicates l)
-  (do ((a '() (if (member (car l) a) a (cons (car l) a)))
-       (l l (cdr l)))
-    ((null? l) (reverse a))))
 (define (make-machine ops controller-text)
   (let* ((machine (make-new-machine))
          (registers-with-dups (filter not-null?
                                       (map extract-register-name controller-text)))
-         (register-names (remove-duplicates registers-with-dups)))
+         (register-names (delete-duplicates registers-with-dups)))
     (for-each (lambda (register-name)
                 ((machine 'allocate-register) register-name))
               register-names)