[DOCS] Remove mention of password protecting the elasticsearch.keystore

[n0othing]

On the Secure Settings page:

https://www.elastic.co/guide/en/elasticsearch/reference/master/secure-settings.html

We mention password protecting the elasticsearch.keystore:

"For this use case, Elasticsearch provides a keystore, which may be password protected, and the elasticsearch-keystore tool to manage the settings in the keystore."

But the ability to password protect hasn't been implemented just yet. To avoid confusion, let's remove the reference for the interim.