[feature/avatars] Get list of enabled drivers from avatar manager

This shouldn't be done in the avatar drivers. We need to force the
display all avatar drivers in the ACP or it won't be possible to enable
avatars after they have been disabled.

PHPBB3-10018

phpBB/includes/avatar/driver/interface.php

@@ -91,13 +91,6 @@ interface phpbb_avatar_driver_interface
 	*/
 	public function delete($row);
 
-	/**
-	* Check if avatar is enabled
-	*
-	* @return bool True if avatar is enabled, false if it's disabled
-	*/
-	public function is_enabled();
-
 	/**
 	* Get the avatars template name
 	*