[Fix] babel's "loose mode" class transform enbrittles BufferList

```js
Object.defineProperty(Object.prototype, util.inspect.custom, {
  set(v) { throw 'this should not happen inside readable-stream'; }
});
```

With this change, I believe the output will use [[Define]] instead of [[Set]] for https://github.com/nodejs/node/blob/c101251a95cc82142bee4637f8db6cc360a06d82/lib/internal/streams/buffer_list.js#L167, and thus no longer fail when Object.prototype is modified.

.babelrc

@@ -13,11 +13,5 @@
       ],
       "debug": true
     }]
-  ],
-  "plugins": [
-    // Included in the preset, but we can't specify per-plugin options there
-    ["@babel/plugin-transform-classes", {
-      "loose": true
-    }]
   ]
 }