Remove unsupported preprocessor-based filters from an adblock list

[ShivanKaul]

We don't currently support preprocessor directives in adblock-rust. This leads to issues when upstream lists push filters that are only meant for Firefox. We should filter out rules that are FF-only on the backend.

chrmod shared https://gist.github.com/chrmod/f26267a9fcd3f39d9adfd401f5c5b168 on our shared Slack which we can adapt pretty easily to filter out rules. We'd want to remove both cap_html_filtering and env_firefox.

[antonok-edm]

I recommend adding the filtering here, where we're already doing another preprocessing step.