[feature/avatars] Properly implement custom HTML in the interface

Previously the driver class added a variable that defined wether an avatar
driver would return custom HTML. The existence of this variable was
implied in the interface. It's also not needed which is why it has been
removed.

PHPBB3-10018

phpBB/includes/avatar/driver/driver.php

@@ -57,13 +57,6 @@ abstract class phpbb_avatar_driver implements phpbb_avatar_driver_interface
 	*/
 	protected $cache;
 
-	/**
-	* This flag should be set to true if the avatar requires a nonstandard image
-	* tag, and will generate the html itself.
-	* @var boolean
-	*/
-	public $custom_html = false;
-
 	/**
 	* Construct a driver object
 	*