man: remove dead code which looked for css

Summary:
This code is not used apparently anymore. The URLs of the two
css files used in the output are either:

• discovered directly (kio_docfilter.css)
• invoked indirectly through help: (kde-default.css)

in the output is discovered separately).

Reviewers: mkoller, marten

Reviewed By: marten

Differential Revision: https://phabricator.kde.org/D6330