Remove hardcoded call to Runtime.exit in JMXKernel

Legacy implementation of bootstrap would manually exit the VM upon server shutdown as a default. With the new bootstrap implmentation, VM-specific stuff is left out of scope.

Because non-daemon Threads are left running in the VM after server shutdown, the VM does not cleanly exit without some help. Therefore JMXKernel.shutdown has a Runtime.exit() directly in it.

This block in the JMXKernel must be removed after server shutdown leaves no non-daemon Threads running.