Cleanup: removed lost synchronized keyword

Original commit: elastic/x-pack-elasticsearch@6eaa2a59a7

src/main/java/org/elasticsearch/alerts/AlertManager.java

@@ -152,9 +152,7 @@ public class AlertManager extends AbstractComponent {
     public void stop() {
         if (started.compareAndSet(true, false)) {
             logger.info("Stopping alert manager...");
-            synchronized (scheduler) {
-                scheduler.stop();
-            }
+            scheduler.stop();
             actionManager.stop();
             alertsStore.stop();
             logger.info("Alert manager has stopped");