Clean up: Drop virtual wiki, finally

Virtual Wiki has been marked "Deprecated" for many years. From the documentations:

```
The Virtual Wiki Features is deprecated altogether.  Don't use it.  It will be removed soon.  Probably before 2010.
See WikiImport and SymbolicLinks as alternative to Virtual Wiki
```

Hence, it seemed about time to get rid of it for real.

* remove Virtual wiki acceptance tests
* remove Extension interface and implementations
* remove anything "Virtual"

Please review. I might have missed something.