Restore permissive error message from permissive/c.

Thanks to @rbfindler for the suggestion.

racket/collects/xml/private/structures.rkt

@@ -51,8 +51,7 @@
          (raise-blame-error
           blame v "not in permissive mode"))))
    #:first-order
-   (lambda (v)
-     (permissive-xexprs))))
+   (lambda (v) #t)))
 
 ; content? : TST -> Bool
 (define content/c