Remove XForms documentation

[duskwuff]

The XForms browser functionality described in features/xforms.xml has never been supported by any major web browser. While there may have been some hints that this would change when this tutorial was written 20 years ago (!!), it's quite clear at this point that XForms isn't going to happen.

(The documentation is also badly outdated - it states that the submitted XML can be found in $HTTP_RAW_POST_DATA, which was removed in PHP 7.0.)