_unlink as opposed to unlink for windows

Change-Id: I40a74c0942594cf48dfbbd3406dc935a4dbfbd35
Reviewed-on: http://review.couchbase.org/40280
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>