Remove plugins dir reference from docs (#46047)

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 #45889

Author: rjernst

docs/reference/upgrade/set-paths-tip.asciidoc

@@ -2,8 +2,7 @@
 ================================================
 
 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.