PLATFORM-10554 Remove diverging paths for CSS sanitization

[slayful]

In upstream MW 1.43, there was a change allowing the filter in style.

This only affects Gamepedia wikis right now:

• When building UCP, we’ve introduced a branch in logic because Gamepedia wikis allowed var and Fandom did not (see: https://fandom.atlassian.net/browse/GPUCP-352).
• During this upgrade, Fandom branch in logic was not affected by MW 1.43.

We can safely change the Fandom branch in logic to allow filter, in MW 1.39 Upgrade we’ve allowed var on Fandom as we pulled in our MW contribution: https://phabricator.wikimedia.org/T288201, so I’d rather remove this branch in logic altogether. That will reduce unnecessary complexity and address the issue.

This reverts commit c47b14f which introduces the branches in logic, making filter correctly work on all wikis.