Fix https://github.com/logstash/expire-logs/issues/14

Remove —keep-open-space option.  Once an index is closed the status can’t tell its size anymore.  If you can’t tell its size anymore, you can’t tell if it needs deleting.  Removing this before it can get scary.
Merge pull request #15 from untergeek/master

Fix https://github.com/logstash/expire-logs/issues/14