[BridgeAbstract] replace getCacheDuration method by CACHE_TIMEOUT class constant

Signed-off-by: Pierre Mazière <pierre.maziere@gmx.com>

lib/BridgeInterface.php

@@ -1,7 +1,6 @@
 <?php
 interface BridgeInterface {
 	public function collectData();
-	public function getCacheDuration();
 	public function getName();
 	public function getURI();
 }