Remove `read_header` option from `CsvReader`, no longer used.
Remove `read_header` option from `CsvReader`, no longer used.

Add a comment with `set_required_header({})` as the idiomatic way of specifying
a present but empty set of required fields.

PiperOrigin-RevId: 426078757

google/riegeli,google/riegeli,google/riegeli,google/riegeli