bridges: Remove getCacheDuration if default

BridgeAbstract will return 3600 seconds by default, so the function
can be removed from any bridge implementing getCacheDuration
returning the same value.

Documentation updated accordingly.

bridges/KoreusBridge.php

@@ -46,8 +46,4 @@ class KoreusBridge extends BridgeAbstract{
 	public function getURI(){
 		return 'http://www.koreus.com/';
 	}
-
-	public function getCacheDuration(){
-		return 3600; // 1 hour
-	}
 }