Drop kio_docfilter.css, no longer used
Depends on these documentation KIO workers aligned with current KDE documentation styling:
- man: !346 (merged)
- info: !347 (merged)
- perldoc: sdk/kdesdk-kio!5 (merged)
Once in, the following holds:
man, info & perldoc KIO workers as the only known consumers all have been
switched to use just kdoctools resources.