MB-100: Remove unused cursorExists function

Change-Id: Ifb1b03594f034a13688f94726538a5b651b4d3bc
Reviewed-on: http://review.couchbase.org/29111
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>