remove unused method. close() is neither needed nor called anywhere

closes #10175

src/main/java/org/elasticsearch/cluster/ClusterStateObserver.java

@@ -147,13 +147,6 @@ public class ClusterStateObserver {
         }
     }
 
-    public void close() {
-        if (observingContext.getAndSet(null) != null) {
-            clusterService.remove(clusterStateListener);
-            logger.trace("cluster state observer closed");
-        }
-    }
-
     /**
      * reset this observer to the give cluster state. Any pending waits will be canceled.
      *