Remove unused import of removed api type

The RollbackConfig type from axtension v1beta1 has
been removed from the k8s api. This import is not
used anyways so removal should be safe.

src/config_types.ts

@@ -1,6 +1,5 @@
 import * as fs from 'fs';
 import * as u from 'underscore';
-import { ExtensionsV1beta1RollbackConfig } from './api';
 
 export interface Cluster {
     readonly name: string;