`@unmacro` regression

[Werner LEMBERG]

In the texinfo manual of version 7.1 I can find the following about
`@unmacro`:


   You can undefine a macro FOO with ‘@unmacro FOO’.  It is not an
   error to undefine a macro that is already undefined.  For example:

     @unmacro foo


However, this doesn't work.  This input document

``` u.texi
\input texinfo

@unmacro foo

@bye
```

processed with `texi2pdf` aborts:

```
./u.texi:3: Macro foo not defined.
@\unmacro ...se @errmessage {Macro #1 not defined}
                                                  @fi 
l.3 @unmacro foo
```


    Werner