MB-100: Remove some unused variables from EventuallyPersistentStore

Change-Id: Ib5b88d39b3cd1489defa68dfaf0bdd399c7b17e8
Reviewed-on: http://review.couchbase.org/25997
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>