Remove unused functions from checkpoint manager and item pager

Change-Id: I09788ffd82e15559a63f5e6f7a8e1a98c28af497
Reviewed-on: http://review.couchbase.org/42408
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>