Remove special treatment for custom vocab files
# What does this PR do?

This PR follows up from #10624 and removes the ability to specify a custom vocab file that doesn't lie in the model repo for a tokenizer (which prevents using the versioning system for those tokenizers).

It also cleans up a tiny bit the `from_pretrained` method, mainly:
- use f-strings instead of `.format()`
- remove double try since you can have several except in cascade
- use a FutureWarning for a deprecation warning that was just sent to the logs and set an end date