Remove ERROR log on non-CloseableServant (Fix #11378)

Since the refactoring the close methods out of
`AbstractAmdServant` and into `CloseableServant`,
an ERROR has been printed for each stateless
service.

As mentioned in the ticket, without this patch
```
import omero
cli=omero.client('localhost')
sess=cli.createSession('root','ome')
sess.getAdminService()
cli.__del__()
```
suffices to print an ERROR to var/log/Blitz-0.log. With
it, there should be none.

See: http://trac.openmicroscopy.org.uk/ome/ticket/11378#comment:6