[hotfix][rest] Remove unnecessary instanceof check

flink-runtime/src/main/java/org/apache/flink/runtime/rest/handler/taskmanager/AbstractTaskManagerFileHandler.java

@@ -123,11 +123,7 @@ public abstract class AbstractTaskManagerFileHandler<M extends TaskManagerMessag
 			blobKeyFuture = fileBlobKeys.get(taskManagerId);
 		} catch (ExecutionException e) {
 			final Throwable cause = ExceptionUtils.stripExecutionException(e);
-			if (cause instanceof RestHandlerException) {
-				throw (RestHandlerException) cause;
-			} else {
-				throw new RestHandlerException("Could not retrieve file blob key future.", HttpResponseStatus.INTERNAL_SERVER_ERROR, e);
-			}
+			throw new RestHandlerException("Could not retrieve file blob key future.", HttpResponseStatus.INTERNAL_SERVER_ERROR, cause);
 		}
 
 		final CompletableFuture<Void> resultFuture = blobKeyFuture.thenAcceptAsync(