docs: pdfdocs: Refactor config for CJK document

To make generated LaTeX code portable across systems with different sets
of available fonts, convert font-availability check in python code to
LaTeX code by using a conditional command provided by the "fontspec"
package.

This will help those who run Sphinx on one machine/container and run
latexmk on other machines/containers.

Remove import of check_output as it is unused any more.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/4baf88af-316d-8aee-240f-6ccbc20907d7@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>