Move charset audit from Best Practice to Performance insight

As discussed with @paulirish crrev.com/c/7577714 adds a Performance Insight for this audit for DevTools. Let's retire the Lighthouse audit to remove code duplication.

As part of that, and since Insights can only be used Performance audits, we'll need to move this audit from Best Practice to Performance category. There's good reasons for it to sit in either so think that is fine.