Fix out-of-date Javadocs on Security.java

We have made the security manager non-optional, but the Javadocs for
Security.java imply that it still is. This commit fixes this issue.

Relates #16176

Author: jasontedor

core/src/main/java/org/elasticsearch/bootstrap/Security.java

@@ -85,12 +85,6 @@
  * cleanups to the scripting apis). But still it can provide some defense for users
  * that enable dynamic scripting without being fully aware of the consequences.
  * <br>
- * <h1>Disabling Security</h1>
- * SecurityManager can be disabled completely with this setting:
- * <pre>
- * es.security.manager.enabled = false
- * </pre>
- * <br>
  * <h1>Debugging Security</h1>
  * A good place to start when there is a problem is to turn on security debugging:
  * <pre>