Removing 'utf8_bom' setting value requirement

When exporting CSV from views via the `views_data_export`, the generated files are receiving no BOM. This is happening because the `csv_serialization` module is requiring that a 'utf8_bom' setting value be provided. However, all the `csv_serialization` module does with this value is confirm that it was passed; i.e. at no point does `csv_serialization` actually use the value, and is instead using BOM_UTF8 constant from vendor/league/csv/src/AbstractCsv.php. This means that even though `views_data_export` is correctly specifying 'utf8' for the 'encoding' setting, the resulting file is not properly encoded. Withal, this patch removes the 'utf8_bom' setting requirement.

Note that the 'utf8_bom' setting being a requirement appears to have come from issue 2859030 → .