Remove `options.filename` from `css.stringify`

Why?

- It is only used to set the `file` property of the source map, and
  that’s probably done just because the `file` property used to be a
  required property of source maps. It isn’t no more.
- The `file` property is not used by browsers.
- The user could easily just add the property himself on the returned
  source map object.
- It was neither documented nor tested.
- KISS.

@conradz:

> I think if we decide to deprecate filename, we can just remove it
> altogether and bump major version.

https://github.com/reworkcss/css-stringify/pull/42#issuecomment-44884423