Remove hasNextForPersistence()

We no longer need this function because the hasNext() function can
be used to figure out if the persistence cursor has another item.

Change-Id: Idf315e4922f1d1c1e6fc7401b5477ae5ef824978
Reviewed-on: http://review.couchbase.org/41506
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>