[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/driver.php

@@ -127,16 +127,6 @@ abstract class phpbb_avatar_driver implements phpbb_avatar_driver_interface
 		return true;
 	}
 
-	/**
-	* @inheritdoc
-	*/
-	public function is_enabled()
-	{
-		$driver = preg_replace('#^phpbb_avatar_driver_#', '', get_class($this));
-
-		return $this->config["allow_avatar_$driver"];
-	}
-
 	/**
 	* @inheritdoc
 	*/