JMX subclass permission check redundant with strong encapsulation

JMX uses RuntimePermission to check (if Security Manager is present) that code is permitted to subclass certain classes.

This SecurityManager check should be removed as unnecessary now we have strong encapsulation with modules.

AgentProvider, PlatformMBeanProvider and PlatformMBeanFinder use a check for RuntimePermission with a name: class.getName() + ".subclass".

Strong encapsulation ensures that no user-defined class can implement those abstract classes.