Remove session state from onClose only

Doing the same eagerly from onError has the side effect of not being able
to delegate handleConnectionClosed to interceptors.

Closes gh-872

spring-graphql/src/main/java/org/springframework/graphql/server/webmvc/GraphQlWebSocketHandler.java

@@ -326,14 +326,6 @@ public class GraphQlWebSocketHandler extends TextWebSocketHandler implements Sub
 		}
 	}
 
-	@Override
-	public void handleTransportError(WebSocketSession session, Throwable exception) {
-		SessionState info = this.sessionInfoMap.remove(session.getId());
-		if (info != null) {
-			info.dispose();
-		}
-	}
-
 	@Override
 	public void afterConnectionClosed(WebSocketSession session, CloseStatus closeStatus) {
 		String id = session.getId();