Remove reference to removed class ClaimSaver

In 3597ddf7cba6e5e6f8f258bd629b9e41adc21278, the class ClaimSaver
was removed. This removes its entry in Wikibase.classes.php.

Change-Id: I8b6314c7b56ac82965bfe84ab7a573555e756b50
Merge "Remove reference to removed class ClaimSaver"