MB-12226: Handle ENOENT error during unlink

If the file is not found during an unlink, it shouldn't be added
to the pending file deletions queue.

Change-Id: Ief306277bbbc946ae18e39dd4819f811f12ea76c
Reviewed-on: http://review.couchbase.org/42173
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Sriram Ganesan <sriram@couchbase.com>