Improve clean-jars when dealing with symbolic links. [LUCENE-6438]

Ever since I started seeing jars in the lib folders use symbolic links on linux I've run into jar problems when working with an old checkout or switching branches on a git checkout. You would normally expect ant clean-jars to help, but it didn't and led to some headaches and random bs.

Turns out, clean-jars is not properly removing all symbolic links for me. I've seen two cases - symbolic links to jars that are not removed and broken symbolic links to jars.

I can get rid of the symbolic links with the following:

```Java



```

But that doesn't work with the broken links.

I guess you can remove those with the Ant Symlink task, but it seems only specifically one at a time which is not that useful.

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Migrated from [LUCENE-6438](https://issues.apache.org/jira/browse/LUCENE-6438) by Mark Miller (@markrmiller), updated May 13 2016
Attachments: [LUCENE-6438.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-6438/LUCENE-6438.patch)
Linked issues:
- [SOLR-7429](https://issues.apache.org/jira/browse/SOLR-7429)
- #7136