SOLR-15215: SolrJ: Remove Netty dependency

[dsmiley]

Netty is an optional dependency of Zookeeper; one must opt-in to use it. Netty remains a solr-core dependency transitively via Hadoop/HDFS.

https://issues.apache.org/jira/browse/SOLR-15215

[janhoy]

I put them in https://issues.apache.org/jira/browse/SOLR-13665. And the reason was to enable secure connections with Zookeeper.

• To be able to use secure solr-zk communication with SSL, netty libs are needed on client side
• Solr hosts themselves use solrj to talk to zookeeper, so they need to be available on the host

See https://issues.apache.org/jira/browse/SOLR-7893 for plans to document how to enable this. If netty is not shipped with Solr, then this needs to be a plugin. Actually I think this would make a good candidate for a package "solrj-zk-netty", which would simply contain the needed jar files and make them available to solrj, which can then talk SSL with Zookeeper.

Problem with removing it before we have an alternative ready is that people already rely on it, see https://issues.apache.org/jira/browse/SOLR-14027 which is a bug report about Admin UI ZK screen not working when using SSL against Zookeeper.

[itygh]

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