Improve bert-japanese tokenizer handling
remove an inelegant string-testing hack in favor of the tooling we now have (support for `config.tokenizer_class`, and versioning of model files in the hub):

see commits enabling this on huggingface.co side:
- https://huggingface.co/yosuke/bert-base-japanese-char/commit/e8365f5c923b98b8cfbf258cfeb14ac536477a31
- https://huggingface.co/daigo/bert-base-japanese-sentiment/commit/88b611269eb04ce60afa4d448b27ffee0a48f5a0
- https://huggingface.co/bandainamco-mirai/distilbert-base-japanese/commit/f411ce0e53839adab9e39187ef179e3b5c836f7c