[tokenizer] sanitize saved config

[stas00]

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

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[sgugger]

Thanks for fixing this!