consider to remove DocSet.close() [LUCENE-7516]

@romseygeek, I'd like to do the subj but I've found

```Java
/** FUTURE: for off-heap */
`@Override`
public void close() throws IOException {
}
```

and want to sync up with @yonik. If he confirm, let's nuke it from master, keeping in 6x as well it's public method. it should be SOLR ticket for sure

---
Migrated from [LUCENE-7516](https://issues.apache.org/jira/browse/LUCENE-7516) by Mikhail Khludnev (@mkhludnev), updated Oct 22 2016
Parent: #8563

Start by locating DocSet.close() and reviewing parent issue #8563 and the referenced LUCENE-7516 context. Confirm with the mentioned maintainers whether it should be removed from master while retained in 6.x, and coordinate the Solr ticket; done means the agreed API changes are reflected without breaking callers.

Written by the indexing model from the issue text.