suppress orb sutdown warning

When running on an ORB that is shut down in another thread, the ORBRunner thread exit will whine:

WARN [com.arjuna.orbportability.logging.loggerI18N] (Thread-19) [com.arjuna.orbportability.OA.caughtexception] shutdownObject caught: org.omg.CORBA.OBJECT_NOT_EXIST: POA destroyed

at com.arjuna.orbportability.RootOA.shutdownObject(RootOA.java:288)
at com.arjuna.ats.internal.jts.OTSImpleManager.purge(OTSImpleManager.java:187)
at com.arjuna.ats.internal.jts.lifecycle.ShutdownOTS.work(ShutdownOTS.java:73)
at com.arjuna.orbportability.RootOA.destroy(RootOA.java:87)
at com.arjuna.ats.internal.jts.orbspecific.jacorb.recoverycoordinators.ORBRunner.run(ORBRunner.java:79)

We don't detect ORB shutdown internally, so do the cleanup unconditionally rather than only if the ORB is still running. Some resulting errors are already silently ignored. This one should be too - it's easier than adding lifecycle tracking to the orb portability layer.