XHTML parser shouldn't run the HTML Cleaner

It's supposed to get valid XHTML as input. Otherwise you should use the HTML parser which performs cleaning.
We had this because before we had a xhtml macro and it was a pain for users to enter valid XHTML (with valid xml header, etc). However we've removed that macro and kept only the html macro so it's fine now.

This allows users of the XHTML parser to do their own cleaning if they want or not to do any cleaning.