remove unused updater

Signed-off-by: Sebastian Schepens <[email protected]>

Author: sschepens

server/src/main/java/io/envoyproxy/controlplane/server/DiscoveryRequestStreamObserver.java

@@ -28,8 +28,6 @@
 public abstract class DiscoveryRequestStreamObserver implements StreamObserver<DiscoveryRequest> {
   private static final AtomicLongFieldUpdater<DiscoveryRequestStreamObserver> streamNonceUpdater =
       AtomicLongFieldUpdater.newUpdater(DiscoveryRequestStreamObserver.class, "streamNonce");
-  private static final AtomicIntegerFieldUpdater<DiscoveryRequestStreamObserver> isClosingUpdater =
-      AtomicIntegerFieldUpdater.newUpdater(DiscoveryRequestStreamObserver.class, "isClosing");
   private static final Logger LOGGER = LoggerFactory.getLogger(DiscoveryServer.class);
 
   final String defaultTypeUrl;