Respect --no-inference when converting xlsx

I have an xlsx file where the `c.internal_value` is not at all what it should be (`54601` instead of `E7C 3G7`, for example). Using `c.value` if the `--no-inference` flag is set solves this issue.

By the way, `'type_inference'` is used nowhere, so I normalized it to `'no_inference'` which is used elsewhere.