Always export CSV encoded in UTF-8
Following https://discuss.tryton.org/t/sao-exports-with-accents/3066/3
The browsers do not provide a way to download the CSV files in other encoding than UTF-8. So I propose to remove the non-working option and to add the UTF-8 BOM[1] to the CSV file on Windows (as program like Excel use it as a magic number).
[1] https://en.wikipedia.org/wiki/Byte_order_mark