Remove allow running as root

This commit removes the escape hatch for running Elasticsearch as root.

Relates #18694

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

@@ -86,11 +86,7 @@ final class Bootstrap {
 
         // check if the user is running as root, and bail
         if (Natives.definitelyRunningAsRoot()) {
-            if (Boolean.parseBoolean(System.getProperty("es.insecure.allow.root"))) {
-                logger.warn("running as ROOT user. this is a bad idea!");
-            } else {
-                throw new RuntimeException("don't run elasticsearch as root.");
-            }
+            throw new RuntimeException("can not run elasticsearch as root");
         }
 
         // enable secure computing mode