MB-12091: Remove usage of unlink in merge sort

Remove the usage of unlink in the merge sort function
as the behavior of unlink is not consistent across all
platforms.

Change-Id: Ib0d9f2514272ce81c5807746e0cbbf1561912883
Reviewed-on: http://review.couchbase.org/41181
Tested-by: Sriram Ganesan <sriram@couchbase.com>
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>