remove unk_token : null value to prevent issues with downstream tools

Most tools ignore the null value but a few tools expect a string value or no value at all and this prevents them from loading the model correctly

tokenizer_config.json

@@ -234,6 +234,5 @@
   "eos_token": "|||IP_ADDRESS|||",
   "model_max_length": 2048,
   "pad_token": "<|padding|>",
-  "tokenizer_class": "GPTNeoXTokenizer",
-  "unk_token": null
+  "tokenizer_class": "GPTNeoXTokenizer"
 }