[tokenizer] sanitize saved config
this PR fixes tokenizer's `save_pretrained` to remove the `name_or_path` entry from `tokenizer_config.json` because:
1. it usually contains the local path that was used to save the model, which is not only invalid once published on the hub, it could potentially reveal some personal information.
2. it is not used anywhere, since one needs to know `name_or_path` before they can load this file.

it also adjusts one tokenizer test not to test for the `name_or_path` entry 
_The documentation is not available anymore as the PR was closed or merged._