jvm: fix memory pool stats registration (#1160)

When I added the condition for ensuring we were still registering
memory pools, I missed dropping the short-circuit return, so we
accidentally dropped these.

spectator-ext-jvm/src/main/java/com/netflix/spectator/jvm/Jmx.java

@@ -50,7 +50,6 @@ public static void registerStandardMXBeans(Registry registry) {
         return t;
       });
       JavaFlightRecorder.monitorDefaultEvents(registry, executor);
-      return;
     } else {
       monitorClassLoadingMXBean(registry);
       monitorThreadMXBean(registry);