Remove plugins dir reference from docs
While the plugin installation directory used to be settable, it has not
been so for several major versions. This commit removes a lingering
reference to the plugins directory in upgrade docs.
closes
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When you extract the zip or tarball packages, the `elasticsearch-n.n.n`
-directory contains the Elasticsearch `config`, `data`, `logs` and
-`plugins` directories.
+directory contains the Elasticsearch `config`, `data`, and `logs` directories.
We recommend moving these directories out of the Elasticsearch directory
so that there is no chance of deleting them when you upgrade Elasticsearch.
