Remove references to FB's HTML() function

When Facebook was just starting to use XHP and transitioning over all
of its HTML strings, it needed a way to migrate non-upgraded code to
work with XHP. The HTML() function was a way to basically just bypass
XHP validation and render strings as HTML. We don’t need to use this
anymore and it never should have been in the open source version
anyways.

README.textile

@@ -55,8 +55,6 @@ echo <div>Hello {$_GET['name']}</div>;
 
 As you can see, using XHP makes safety the default rather than the exception.
 
-If for some reason you need to bypass XHP's escaping you can explicitly upcast a raw-str to an html-str with the @HTML()@ function. For example: @<div>{HTML($html)}</div>@. You should avoid this at all costs, however. See the "Best Practices" section below for more information.
-
 h1. Defining Elements
 
 All elements in XHP are just PHP classes. Even the basic HTML elements like div and span are classes. You define an element just like you do a class, except you use a leading colon to specify that you're creating an XHP element:
@@ -156,8 +154,6 @@ class :fb:thing extends :x:element {
 
 This element would be considered magic because when you print an @<fb:thing />@ it actually returns a div.
 
-* Don't use @HTML()@. You should not use @HTML()@ unless you have a very good reason, and you have a very very good understanding of the differences between HTML strings and raw strings. And even then, avoid @HTML()@.
-
 h1. External Resources
 
 Below are a list of external resources about XHP:

php-lib/core.php

@@ -49,8 +49,6 @@ protected static function &__xhpAttributeDeclaration() {}
   final protected static function renderChild($child) {
     if ($child instanceof :xhp) {
       return $child->__toString();
-    } else if ($child instanceof HTML) {
-      return $child->render();
     } else if (is_array($child)) {
       throw new XHPRenderArrayException('Can not render array!');
     } else {
@@ -617,11 +615,6 @@ final protected function validateChildren() {
     $ii = 0;
     if (!$this->validateChildrenExpression($decl, $ii) ||
         $ii < count($this->children)) {
-      // Use of HTML() breaks the content model definition.
-      // Lesson: Don't use HTML().
-      if (isset($this->children[$ii]) && $this->children[$ii] instanceof HTML) {
-        return;
-      }
       throw new XHPInvalidChildrenException($this, $ii);
     }
   }
@@ -855,7 +848,7 @@ final public function __toString() {
       }
 
       return $that->__toString();
-      
+
     } catch (Exception $error) {
       trigger_error($error->getMessage(), E_USER_ERROR);
     }