fix(special_tokens_map): Removes unk_token.

special_tokens_map.json

@@ -19,12 +19,5 @@
     "normalized": false,
     "rstrip": true,
     "single_word": false
-  },
-  "unk_token": {
-    "content": "�",
-    "lstrip": false,
-    "normalized": false,
-    "rstrip": false,
-    "single_word": false
   }
 }