Remove now deprecated / ignored xla_device config setting

Avoid
```
The `xla_device` argument has been deprecated in v4.4.0 of Transformers. It is ignored and you can safely remove it from your `config.json` file.
```

config.json

@@ -48,5 +48,4 @@
     }
   },
   "vocab_size": 32128,
-  "xla_device": true
 }