Update ClassLoaderLeakPreventor.java

If you enable jmx support in jetty 8 or 9  some mbeans (e.g. for the servletholder or sessionmanager) are instanciated in the web application thread  and a reference to the WebappClassloader is stored in a private ObjectMBean._loader which is unfortunatly not the classloader that loaded the class.
So for unregisterMBeans to work even for the jetty mbeans we need to access the MBeanContainer class of the jetty container to remove the leaking mbeans.