Revert deprecation of Object::add_extension() usage

This reverts commit 14b997eea3.
Its just not practical to use the Config API as it stands,
the add_extension() wrapper does more than just a Config->update().

Most use cases can be covered via YML, but any conditional
additions (e.g. in unit tests) can still benefit from the
add_extensions() shorthand.

core/Object.php

@@ -440,24 +440,23 @@ abstract class Object {
 	
 	/**
 	 * Add an extension to a specific class.
+	 *
+	 * The preferred method for adding extensions is through YAML config,
+	 * since it avoids autoloading the class, and is easier to override in
+	 * more specific configurations.
+	 * 
 	 * As an alternative, extensions can be added to a specific class
 	 * directly in the {@link Object::$extensions} array.
 	 * See {@link SiteTree::$extensions} for examples.
 	 * Keep in mind that the extension will only be applied to new
 	 * instances, not existing ones (including all instances created through {@link singleton()}).
 	 *
-	 * @deprecated 3.2 Use YAML config instead, see 
-	 *             http://doc.silverstripe.org/framework/en/trunk/reference/dataextension
+	 * @see http://doc.silverstripe.org/framework/en/trunk/reference/dataextension
 	 * @param string $class Class that should be extended - has to be a subclass of {@link Object}
 	 * @param string $extension Subclass of {@link Extension} with optional parameters 
 	 *  as a string, e.g. "Versioned" or "Translatable('Param')"
 	 */
 	public static function add_extension($classOrExtension, $extension = null) {
-		Deprecation::notice(
-			'3.2', 
-			'Use YAML config instead, see http://doc.silverstripe.org/framework/en/trunk/reference/dataextension'
-		);
-
 		if(func_num_args() > 1) {
 			$class = $classOrExtension;
 		} else {

docs/en/changelogs/3.1.0.md

@@ -450,4 +450,3 @@ you can enable those warnings and future-proof your code already.
  * Hard limit displayed pages in the CMS tree to `500`, and the number of direct children to `250`,
    to avoid excessive resource usage. Configure through `Hierarchy.node_threshold_total` and `
    Hierarchy.node_threshold_leaf`.  Set to `0` to show tree unrestricted.
- * Deprecated `Object::add_extension()`, use YAML config instead, see [docs](http://doc.silverstripe.org/framework/en/trunk/reference/dataextension)

docs/en/reference/dataextension.md

@@ -33,12 +33,9 @@ In order to active this extension, you'd add the following to your [config.yml](
 	    - MyMemberExtension
 
 Alternatively, you can add extensions through PHP code as well (in your `config.php` file), 
-which means they can be used in conditional configuration.
+which means they can be used in conditional configuration. 
 
 	:::php
-	// Preferred notation: Through the Config API
-	Config::inst()->update('Member', 'extensions', array('MyMemberExtension'));
-	// Legacy notation: Through static class access
 	Member::add_extension('MyMemberExtension');
 
 ##  Implementation