Remove dead SecurityManager code (JEP 486)

System.getSecurityManager() always returns null since Java 24.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

common/util/src/main/java/org/thingsboard/common/util/ThingsBoardThreadFactory.java

@@ -33,9 +33,7 @@ public class ThingsBoardThreadFactory implements ThreadFactory {
     }
 
     private ThingsBoardThreadFactory(String name) {
-        SecurityManager s = System.getSecurityManager();
-        group = (s != null) ? s.getThreadGroup() :
-                Thread.currentThread().getThreadGroup();
+        group = Thread.currentThread().getThreadGroup();
         namePrefix = name + "-" +
                 poolNumber.getAndIncrement() +
                 "-thread-";