maintenance: Remove cssjanus.py

Follows-up r55510 (aa4d684a82) which added it for use with the
Vector stylesheets at times where we didn't have CSSJanus.php.

This has been unused since we introduced ResourceLoader and the
PHP-port of CSSJanus.

Change-Id: I2c9a022354617060d99bf1c44f0726b495ec070d